Tag Archives: system of equations

Exploring Sequences and Lines

Here’s another favorite problem that could be used for any middle or high school students who’ve been exposed to both arithmetic sequences and linear equations.

There is a family of lines, Ax+By+C=0, for which \left\{ A, B, C \right\} can be any arithmetic sequence.  What do all members of this family have in common?

As with most great math problems, the problem is simply stated and can be approached from many different perspectives.  In the end, this one isn’t that difficult to crack, but the solution (at least to me) is not intuitively obvious from the problem statement. If you’ve not seen this before, please try it before reading further.

WARNING:  Problem Solution Follows

MOST COMMON STUDENT APPROACH:  Given the problem’s phrasing, most begin by writing out a few possible equations using different arithmetic sequences, typically with positive common differences.  After a few tries, most get a little frustrated as no obvious pattern emerges from the equations.

It is this point that is typically the most difficult for me as a teacher.  I want to help anyone who struggles, but “help” often means allowing others the room to struggle and to find ways of solving problems on their own.  Only when a student or group gets really frustrated do I sometimes ask, “Are there any other ways you can look at this problem or at your work?”

Eventually, most decide to graph their equations to see if anything pops out.  The following graph was submitted this past semester by one of my precalculus students using the free online Desmos calculator.

ALines1

Two lines intersecting in a point is common.  Three or more in a single point almost always indicates something interesting.  Seven lines through a single point is screaming for attention!  From this graph, all lines in this family apparently contain the point (1,-2).  That seems a bit random until you investigate further, but pattern recognition is just half of the battle for a mathematician.  Now that something interesting has been discovered, a mathematician wants to know if this is a fluke or something inherent to all members of the family.

NOTE:  All graphs my students have produced over the years have always contained specific equations.  I don’t know that it’s any more enlightening, but I did create on Desmos a slider version of the graphs of this family with one slider for the initial term (A) and a second for its common difference (d).

UNIQUE SOLUTION METHODS FROM STUDENTS:

All successful solutions eventually rewrite the sequence \left\{ A,B,C \right\} to \left\{ A,A+d,A+2d \right\} where d is the common difference for a generic arithmetic sequence with initial term A.

Method I: After discovering the common point of intersection, most plug it into the left side of the equation and simplify to get

Ax+By+C=A\cdot 1+\left( A+d\right)\cdot -2+\left( A+2d \right)=0.

Because the left side reduces to zero for all generic arithmetic sequences, \left\{ A,A+d,A+2d \right\}, (1,-2) must be common to all members of this family.

A few students aren’t comfortable dealing with 0=0, so these tend to plug in x=1 and solve for y to get y=-2, proving that the y-coordinate for x=1 for all members of this family is always -2.

Method II:  A few students every year work algebraically from properties of arithmetic sequences.  For any arithmetic sequence, \left\{ A,B,C \right\}, \frac{A+C}{2}=B.  This rewrites to 1\cdot A-2\cdot B+C=0, so whenever \left( x,y \right)=\left(1,-2 \right), then Ax+By+C=0 is a fundamental property of all arithmetic sequences.

Personally, I think this method gets closest to explaining why the point (1,-2) is the common characteristic of this family.

Method III:  This year, I had a student take an approach I’d never seen before.  She defined one arithmetic sequence as \left\{ a,a+d,a+2d \right\} and another as \left\{ m,m+n,m+2n \right\} for any real values of a,d,m, and n.  This leads to a system of equations: a\cdot x+(a+d)\cdot y+(a+2d)=0 and m\cdot x+(m+n)\cdot y+(m+2n)=0 .  If you have some younger students or if all the variables make you nervous, the solution is available via Wolfram Alpha.

Still, this system is pretty easy to manipulate.  If you multiply the first equation by m and the second by a, the x-terms will eliminate with subtraction, giving

m\cdot((a+d)\cdot y+(a+2d))-a\cdot((m+n)\cdot y+(m+2n))=0.

Solving for y shows that all of the coefficients simplify surprisingly easily.

((ma+md)-(am+an))\cdot y=-(ma+2md)+(am+2an)
(md-an)\cdot y = -2\cdot (md-an)\Longrightarrow y=-2

From here, determining x=1 is easy, proving the relationship.

SOLUTIONS and APPROACHES NOT YET OFFERED BY STUDENTS:

Approach A:  High school students don’t often think about ways to simplify problem situations, especially at the beginning of problems.  One way I did that for this problem in later class discussions was to recognize that it one of the terms in the arithmetic sequence was 0, you didn’t need to deal with nearly as many terms. For example, if your sequence was {1,0,-1}, the linear equation would be x-1=0.  Similarly, the sequence \left\{ 01,2 \right\} leads to y+2=0.  Obviously, the only thing these two lines have in common is the point (1,-2).  A proof of the property must still be established, but this is one of the fastest ways I’ve seen to identify the central property.

Approach B:  A purely algebraic approach to this problem could redefine the arithmetic sequence as \left\{ a,a+d,a+2d\right\}as before, giving:

a\cdot x+(a+d)\cdot y+(a+2d)=0

Collecting like terms gives

(x+y+1)\cdot a+(y+2)\cdot d=0.

The values of a and d must remain as parameters to include all possible arithmetic sequences.  Because the equation always equals 0, the coefficients of a and d are both 0, making y=-2 (for the coefficient of d) and therefore x=1.

EXTENSION:

We once had a test question at the end of the unit containing this exercise.  Basically, it reminded students that they had discovered that all lines Ax+By+C=0, for which \left\{ A, B, C \right\} can be any arithmetic sequence contained the point (1,-2).  It then asked for an equation of a family of linear functions using the same arithmetic \left\{ A, B, C \right\} that all contained the point (1,2).

The two most common responses we’ve seen involve a reflection or a vertical translation.  (1,-2) can become (1,2) by reflecting over the x-axis, so making the y-values negative would do the trick:  Ax-By+C=0.  Similarly, (1,-2) can become (1,2) by translating up 4 units, giving Ax+B(y-4)+C=0.

Exponentials and Transformations

Here’s an old and (maybe) a new way to think about equations of exponential functions.  I suspect you’ve seen the first approach.  If you understand what exponentials functions are, my second approach using transformations is much faster and involves no algebra!

Members of the exponential function family can be written in the form y=a\cdot b^x for real values of a and postive real values of b.  Because there are only two parameters, only two points are required to write an equation of any exponential.

EXAMPLE 1: Find an exponential function through the points (2,5) and (4,20).  

METHOD 1:  Plug the points into the generic exponential equation to get a 2×2 system of equations.  It isn’t necessary, but to simplify the next algebra step, I always write the equation with the larger exponent on top.

\left\{\begin{matrix} 20=a\cdot b^4 \\ 5=a\cdot b^2 \end{matrix}\right.

If the algebra isn’t the point of the lesson, this system could be solved with a CAS.  Users would need to remember that b>0 to limit the CAS solutions to just one possibility.

If you want to see algebra, you could use substitution, but I recommend division.  Students’ prior experience with systems typically involved only linear functions for which they added or subtracted the equations to eliminate variables.  For exponentials, the unknown parameters are multiplied, so division is a better operational choice.  Using the system above, I get \displaystyle \frac{20}{5}=\frac{a\cdot b^4}{a\cdot b^2}.  The fractions must be equivalent because their numerators are equal and their denominators are equal.

Simplifying gives 4=b^2\rightarrow b=+2 (because b>0 for exponential functions) and a=\frac{5}{4}.

This approach is nice because the a term will always cancel from the first division step, leaving a straightforward constant exponent to undo, a pretty easy step.

METHOD 2:  Think about what an exponential function is and does.  Then use transformations.

Remember that linear functions (y=m\cdot x+b) “start” with a y-value of b (when x=0) and add m to y every time you add 1 to x.  The only difference between linear and exponential functions is that exponentials multiply while linears add.  Therefore, exponential functions (y=a\cdot b^x) “start” with a y-value of a when x=0 and multiply by b every time 1 is added to x.

What makes the given points a bit annoying is that neither is a y-intercept.  No problem.  If you don’t like the way a problem is phrased, CHANGE IT!    (Just remember to change the answer back to the original conditions!)

If you slide the given points left 2 units, you get (0,5) and (2,20).  It would also be nice if the points were 1 x-unit apart, so halving the x-values gives (0,5) and (1,20).  Because the y-intercept is now 5, and the next point multiplies that by 4, an initial equation for the exponential is y = 5\cdot 4^x . To change this back to the original points, undo the transformations at the start of this paragraph:  stretch horizontally by 2 and then move right 2.  This gives y = 5\cdot 4^\frac{x-2}{2}.

This is algebraically equivalent to the y=\frac{5}{4}\cdot 2^x found early.  Obviously, my students prove this.

One student asked why we couldn’t make the (4,20) point the y-intercept.  Of course we can!  To move more quickly through the set up, starting at (4,20) and moving to (2,5) means my initial value is 20 and I multiply by \frac{1}{4} if the x-values move left 2 from an initial x-value of 4.  This gives y = 20\cdot\left( \frac{1}{4} \right) ^\frac{x+4}{-2}.  Of course, this 3rd equation is algebraically equivalent to the first two.

Here’s one more example to illustrate the speed of the transformations approach, even when the points aren’t convenient.

EXAMPLE 2: Find an exponential function through (-3,7) and (12,13).  

Starting at (-3,7) and moving to (12,13) means my initial value is 7, and I multiply by \frac{13}{7} if the x-values move right 15 from an initial x-value of -3.  This gives y = 7\cdot\left( \frac{13}{7} \right) ^\frac{x-3}{15}.

Equivalently, starting at (12,13) and moving to (-3,7) means my initial value is 13, and I multiply by \frac{7}{13} if the x-values move left 15 from an initial x-value of 12.  This gives y = 13\cdot\left( \frac{7}{13} \right) ^\frac{x+3}{-15}.

If you get transformations, exponential equations require almost no algebraic work, no matter how “ugly” the coordinates.  I hope this helps give a different perspective on exponential function equations and helps enhance the importance of the critical math concept of equivalence.

Quadratics, Statistics, Symmetry, and Tranformations

A problem I assigned my precalculus class this past Thursday ended up with multiple solutions by the time we finished.  Huzzah for student creativity!

The question:

Find equations for all polynomial functions, y=f(x), of degree \le 2 for which f(0)=f(1)=2 and f(3)=0.

After they had worked on this (along with several variations on the theme), four very different ways of thinking about this problem emerged.  All were valid and even led to a lesson I hadn’t planned–proving that, even though they looked different algebraically, all were equivalent.  I present their approaches (and a few extras) in the order they were offered in our post-solving debriefing.

The commonality among the approaches was their recognition that 3 non-collinear points uniquely define a vertical parabola, so they didn’t need to worry about polynomials of degree 0 or 1.  (They haven’t yet heard about rotated curves that led to my earlier post on rotated quadratics.)

Solution 1–Regression:  Because only 3 points were given, a quadratic regression would derive a perfectly fitting quadratic equation.  Using their TI-Nspire CASs, they started by entering the 3 ordered pairs in a Lists&Spreadsheets window.  Most then went to a Calculator window to compute a quadratic regression.  Below, I show the same approach using a Data&Statistics window instead so I could see simultaneously the curve fit and the given points.

The decimals were easy enough to interpret, so even though they were presented in decimal form, these students reported y=-\frac{1}{3}x^2+\frac{1}{3}x+2.

For a couple seconds after this was presented, I honestly felt a little cheated.  I was hoping they would tap the geometric or algebraic properties of quadratics to get their equations.  But I then I remembered that I clearly hadn’t make that part of my instructions.  After my initial knee-jerk reaction, I realized this group of students had actually done exactly what I explicitly have been encouraging them to do: think freely and take advantage of every tool they have to find solutions.  Nothing in the problem statement suggested technology or regressions, so while I had intended a more geometric approach, I realized I actually owed these students some kudos for a very creative, insightful, and technology-based solution.  This and Solution 2 were the most frequently chosen approaches.

Solution 2–Systems:  Equations of quadratic functions are typically presented in standard, factored, or vertex form.  Since neither two zeros nor the vertex were explicitly given, the largest portion of the students used the standard form, y=a\cdot x^2+b\cdot x+c to create a 3×3 system of equations.  Some solved this by hand, but most invoked a CAS solution.  Notice the elegance of the solve command they used, working from the generic polynomial equation that kept them from having to write all three equations, keeping their focus on the form of the equation they sought.

This created the same result as Solution 1, y=-\frac{1}{3}x^2+\frac{1}{3}x+2.

CAS Aside: No students offered these next two solutions, but I believe when using a CAS, it is important for users to remember that the machine typically does not care what output form you want.  The standard form is the only “algebraically simple” approach when setting up a solution by hand, but the availability of technology makes solving for any form equally accessible.

The next screen shows that the vertex and factored forms are just as easily derived as the standard form my students found in Solution 2.

I was surprised when the last line’s output wasn’t in vertex form, y=-\frac{1}{3}\cdot \left ( x-\frac{1}{2} \right )^2+\frac{25}{12}, but the coefficients in its expanded form clearly show the equivalence between this form and the standard forms derived in Solutions 1 and 2–a valuable connection.

Solution 3–Symmetry:  Two students said they noticed that f(0)=f(1)=2 guaranteed the vertex of the parabola occurred at x=\frac{1}{2}.  Because f(3)=0 defined one real root of the unknown quadratic, the parabola’s symmetry guaranteed another at x=-2, giving potential equation y=a\cdot (x-3)(x+2).  They substituted the given (0,2) to solve for a, giving final equation y=-\frac{1}{3}\cdot (x-3)(x+2) as confirmed by the CAS approach above.

Solution 4–Transformations:  One of the big lessons I repeat in every class I teach is this:

If you don’t like how a question is posed.  Change it!  
Just remember to adjust the answer.

Notice that two of the given points have the same y-coordinate.  If that y-coordinate had been 0 (instead of its given value, 2), a factored form would be simple.  Well, why not force them to be x-intercepts by translating all of the given points down 2 units?

The transformed data show x-intercepts at 0 and 1 with another ordered pair at (3,-2).  From here, the factored form is easy:  y=a\cdot (x-0)(x-1).  Substituting (3,-2) gives a=-\frac{1}{3} and the final equation is y=-\frac{1}{3}\cdot (x-0)(x-1) .

Of course, this is an equation for the transformed points.  Sliding the result back up two units, y=-\frac{1}{3}\cdot (x-0)(x-1)+2, gives an equation for the given points.  Aside from its lead coefficient, this last equation looked very different from the other forms, but some quick expansion proved its equivalence.

Conclusion:  It would have been nice if someone had used the symmetry noted in Solution 3 to attempt a vertex-form answer via systems.  Given the vertex at x=\frac{1}{2} with an unknown y-coordinate, a potential equation is y=a\cdot \left ( x-\frac{1}{2} \right )^2+k.  Substituting (3,0) and either (0,2)\text{ or }(1,2) creates a 2×2 system of linear equations, \left\{\begin{matrix}  0=a\cdot \left ( 3-\frac{1}{2} \right )^2+k \\  2=a\cdot \left ( 0-\frac{1}{2} \right )^2+k  \end{matrix}\right..  From there, a by-hand or CAS solution would have been equally acceptable to me.

That the few alternative approaches I offered above weren’t used didn’t matter in the end.  My students were creative, followed their own instincts to find solutions that aligned with their thinking, and clearly appreciated the alternative ways their classmates used to find answers.  Creativity and individual expression reigned, while everyone broadened their understanding that there’s not just one way to do math.

It was a good day.

Cubics and CAS

Here’s a question I posed to one of my precalculus classes for homework at the end of last week along with three solutions we developed.

Suppose the graph of a cubic function has an inflection point at (1,3) and passes through (0,-4).

  1. Name one other point that MUST be on the curve, and
  2. give TWO different cubic equations that would pass through the three points.

SOLUTION ALERT!  Don’t read any further if you want to solve this problem for yourself.

The first question relies on the fact that every cubic function has 180 degree rotational symmetry about its inflection point.  This is equivalent to saying that the graph of a cubic function is its own image when the function’s graph is reflected through its inflection point.

That means the third point is the image of (0,-4) when point-reflected through the inflection point (1,3), which is the point (2,10) as shown graphically below.

From here, my students came up with 2 different solutions to the second question and upon prodding, we created a third.

SOLUTION 1:  Virtually every student assumed y=a\cdot x^3 was the parent function of a cubic with unknown leading coefficient whose “center” (inflection point) had been slid to (1,3).  Plugging in the given (0,-4) to (y-3)=a\cdot (x-1)^3 gives a=7.  Here’s their graph.

SOLUTION 2:  Many students had difficulty coming up with another equation.  A few could sketch in other cubic graphs (curiously, all had positive lead coefficients) that contained the 3 points above, but didn’t know how to find equations.  That’s when Sara pointed out that if the generic expanded form of a cubic was a\cdot x^3+b\cdot x^2 +c\cdot x+d , then any 4 ordered pairs with unique x-coordinates should define a unique cubic.  That is, if we picked any 4th point with x not 0, 1, or 2, then we should get an equation.  That this would create a 4×4 system of equations didn’t bother her at all.  She knew in theory how to solve such a thing, but she was thinking on a much higher plane.  Her CAS technology expeditiously did the grunt work, allowing her brain to keep moving.

A doubtful classmate called out, “OK.  Make it go through (100,100).”  Following is a CAS screen roughly duplicating Sara’s approach and a graph confirming the fit.  The equation was onerous, but with a quick copy-paste, Sara had moved from  idea to computation to ugly equation and graph in just a couple minutes.  The doubter was convinced and the “wow”s from throughout the room conveyed the respect for the power of a properly wielded CAS.

In particular, notice how the TI-Nspire CAS syntax in lines 1 and 3 keep the user’s focus on the type of equation being solved and eliminates the need to actually enter 4 separate equations.  It doesn’t always work, but it’s a particularly lovely piece of scaffolding when it does.

SOLUTION 3:  One of my goals in Algebra II and Precalculus courses is to teach my students that they don’t need to always accept problems as stated.  Sometimes they can change initial conditions to create a much cleaner work environment so long as they transform their “clean” solution back to the state of the initial conditions.

In this case, I asked what would happen if they translated the inflection point using T_{-1,-3} to the origin, making the other given point (-1,-7).  Several immediately called the 3rd point to be (1,7) which “untranslating” — T_{1,3}(1,7)=(2,10) — confirmed to be the earlier finding.

For cases where the cubic had another real root at x=r, then symmetry immediately made x=-r another root, and a factored form of the equation becomes y=a\cdot (x)(x-r)(x+r) for some value of a.  Plugging in (-1,-7) gives a in terms of r.

The last line slid the initially translated equation using T_{1,3} to re-position the previous line according to the initial conditions.  While unasked for, notice how the CAS performed some polynomial division on the right-side expression.

I created a GeoGebra document with a slider for the root using the equation from the last line of the CAS image above.  The image below shows one possible position of the retranslated root.  If you want to play with a live version of this, you will need a free copy of GeoGebra to run it, but the file is here.

 

What’s nice here is how the problem became one of simple factors once the inflection point was translated to the origin.  Notice also that the CAS version of the equation concludes with +7x-4, the line containing the original three points.  This is nice for two reasons.  The numerator of the rational term is -7x(x-2)(x-1) which zeros out the fraction at x=0, 1, or 2, putting the cubic exactly on the line y=7x-4 at those points.

The only r-values are in the denominator, so as r\rightarrow\infty, the rational term also becomes zero.  Graphically, you can see this happen as the cubic “unrolls” onto y=7x-4 as you drag |x|\rightarrow\infty.  Essentially, this shows both graphically and algebraically that y=7x-4 is the limiting degenerate curve the cubic function approaches as two of its transformed real roots grow without bound.

 

CAS-ing a triangle

Check out this fun little problem from @daveinstpaul.

I’m sure there is a much more elegant solution, but given my technology interests, I thought this would be a cool way to incorporate CAS.  Based on Dave’s restrictions, the following angle measures apply.

There are many ways to write equations from this setup, several of which are identical forms of the same information.  One way to keep from writing dependent equations is to use combinations.  Using only the smallest triangles, you get

2A+Z=180
2X+Y=180
2W+V=180

From the top two triangles, one relationship is  A+Y=180. Because the large triangle is isosceles, the bottom two triangles give X+V=W.  There’s no convenient way to combine the top and bottom triangle.

Combining all three triangles, one relationship is X+W+Z=180.

That’s a system of 6 equations in 6 variables which Wolfram Alpha solves to give A=45 degrees.

I’d love to see a non-algebraic approach.

Thought Variations and Tests as Learning Tools

I love seeing the different ways students think about solving problems.  Many of my classes involve students analyzing the pros and cons of different approaches.

As an example, a recent question on my first trigonometry test in my precalculus class asked students to find all exact solutions to 3sin^2x-cos^2x=2 in the interval 0\leq x\leq 2\pi.  Admittedly, this is not a complicated problem, but after grading several standard approaches to a solution, one student’s answer (Method 3 below) provided a neat thinking alternative.

As an assessment tool, I don’t view any test as a final product.  While optional, all of my students are encouraged to complete corrections on any test question which didn’t receive full credit.  For me, corrections always require two parts:

  1. Specifically identify the error(s) you made on the problem.
  2. Provide a correct solution to the problem.

My students usually take their tests on their own, but after they are returned, they are encouraged to reference any sources they want (classmates, notes, me, the Web, anyone or anything …) to address the two requirements of test corrections.  The point is for my students to learn from their misunderstandings using any source (or sources) that work for them.  Because students are supposed to do self-assessments, I intentionally don’t provide lots of detail on my initial evaluation of their work.

To show their different approaches, I’ve included the solutions of three students.  Complete solutions  are shown so that you can see the initial feedback I offer.  If there’s interest, I’m happy to provide examples of student test corrections in a future post.

Method 1:  Substitution–By far the most common approach taken.  This student solved sin^2x+cos^2x=1 for sin^2x and substituted.  Others substituted for cos^2x.  [You can click on each image for a full-size view]

This solution started well, but she had an algebra error and an angle identification problem.

Method 2:  Elimination–The same Pythagorean identity could be added or subtracted from the given equation.  After talking yesterday with the student who created this particular solution, I was told that he initially completed the left column and attempted the work in the right column as a check at the end of the period.  After committing the same algebra error as the student in method one, he realized at the end of the test that something was amiss when the cosine approach provided an answer different from the two he initially found using the sine approach.

After conversations with classmates yesterday, he caught his algebra error and found the missing answer.  He also corrected the units issue.

[I’m not sure whether I should even care about the units here and am seriously considering removing the 0\leq x\leq 2\pi restriction from future questions.  With enough use in class, they’ll eventually catch on to radian measure.]

Method 3:  Creation–This approach was used by only one student in the class and uses the same Pythagorean identity.  The difference here is that he initially moved the cos^2x term to the other side and then added an additional 3cos^2x to both sides to create a 3 on the left using the identity.  Nothing like this had been discussed in class, and I was quite interested to learn that the student wasn’t even sure his approach was valid.  What I particularly liked was that this student created an expression in his solution rather than eliminating expressions given in the initial equation as every other student in the class had done.  It reflected a mantra I often repeat in class:  If you don’t like the form of a problem (or want a different form), change it!

Also notice how he used an absolute value in the penultimate line rather than the more common \pm.

Again, nothing especially deep about any of these, but I learn so much from watching how students solve problems.  Hopefully they gain at least as much from each other when comparing each others solutions during corrections.

Nested Isosceles Triangles

Check out this fun little problem from @daveinstpaul.

I’m sure there is a much more elegant solution, but given my technology interests, I thought this would be a cool way to incorporate CAS.  Based on Dave’s restrictions, the following angle measures apply.

There are many ways to write equations from this setup, many of which are identical forms of the same information.  One way to keep from writing dependent equations is to use combinations.

From each small single triangle, you get 2A+Z=180, 2X+Y=180, and 2W+V=180.

From the top two triangles, one relationship is  A+Y=180. Because the large triangle is isosceles, the bottom two triangles give X+V=W.  There’s no convenient way to combine the top and bottom triangle.

Combining all three triangles, one relationship is X+W+Z=180.

That’s a system of 6 equations in 6 variables which Wolfram Alpha solves to give A=45 degrees.

I’d love to see a non-algebraic approach.