Answer the Question that was Asked

I like math contests for many reasons.  Here’s one more reason:

Even the earliest math contests train young students to answer the question that was asked, not the “answer” they came up with.  Here are some examples from Math Olympiad, designed for students in grades 4 – 6.

This question asks for the sum of the 5 prime numbers:moems 3

 

This question asks for the least of these numbers:moems 2

 

This question asks for the sum of two numbers:MOEMS 1

 

When students learn this discipline early, they won’t miss key instructions like this one from a practice SAT:sat mathA student may get the correct estimate for height at 2 years old or 5 years old, but fail to answer the question that was asked.

 

Careful reading requires practice, and is not something a student sees when all they use are worksheets like this:table

5 Ways to Show Your Work

I tutor students who are using the AoPS math curriculum and/or are preparing for math contests like MathCounts and AMC.

Today I’m sharing 5 Ways to Show Your Work in math that you can start doing today.
  1. Line Up Equal Signs.  Nothing helps organize your thoughts like equal signs in a straight line.
  2. Reorient Similar Triangles.  You found similar triangles.  Yay!  The last thing you want now is wrong ratios from wrong corresponding sides.  To avoid errors, redraw the triangles so that corresponding sides are oriented the same way.
  3. Arrows, people!    Marie Kondo your cluttered diagrams with arrows.  Help your readers follow your reasoning.
  4. Line Up Substitutions.  Everything that’s equal is in the same column.
  5. Practice your 3D Drawings and Cross Sections.  Get used to slicing up solids and drawing parallel lines to vanishing points.  It’s art!  It’s math!

Rates

Students who are solid on rate problems will earn easy points on MathCounts!

img_20190702_135510

The only formula to memorize is the definition of rate, r=\frac{d}{t} where d= distance and t= time.  I find it helpful to draw a quick diagram so I can visualize what is happening to our MathCounts team on their way to and from the contest.  (H=home, MC = MathCounts contest.)

On the return trip we multiply the time by 2, multiply the distance by \frac{3}{2}, and subtract 10 mi/h from the rate.

The first equation I have written derives from the fact that the new rate is equal to the old rate minus 10 mi/h.  Then it is a matter of solving for r=\frac{d}{t} which is the rate to the contest.

Source: MathCounts

Number of factors

There’s a neat trick for finding the number of factors of a number.  First find the prime factorization of the number, for example:

756 = 2^2 \cdot 3^3 \cdot 7^1

Add +1 to the power of each prime factor and then multiply those numbers together.  In this example,

(2+1)\cdot(3+1)\cdot(1+1)= 3\cdot 4 \cdot 2 = 24 factors.

This reflects the fact that each factor of 756 contains 2^n where n= 0, 1, or 2 and 3^m, where m = 0, 1, 2, or 3 and 7^p where p = 0 or 1.

I used this property to solve a problem in a number theory class I’m taking.  The problem asks to compute the sum of all positive integers k such that 1984k has  21 positive factors.

Since 21 = 3\cdot7 , working backward we are looking for a prime factorization with 2 primes raised to the powers of 2 and 6.  The prime factorization of 1984 = 2^6\cdot 31 so we have one prime raised to the sixth power.  All we need is the other prime to be squared.  Setting k=31 gives us

1984k = 2^6\cdot 31^2 with (6+1)(2+1)= 21 divisors.

Python for fun

screenshot-2019-05-27-at-3.06.21-pm

With the school year over, I’ve had time to pursue other math related activities.  I’ve been reading through the book Joy of SET by Liz McMahon and others.  It describes the “many mathematical dimensions of a seemingly simple card game” and was recommended to me a few months ago.

There were a few chapters on various probabilities, like the probability of finding 1 set out of 4 randomly chosen cards.  I enjoy writing  python programs that can confirm calculated probabilities by running a simulation several thousands of times.

My program will step through a game of SET, dealing 12 cards, finding SETs and removing them from the table, and dealing more cards so that there are 12 cards on the table again.  If there are no SETs in the dozen cards, then it deals 3 more.

I also wrote a function that will find the probability of finding n SETs when one randomly deals x cards, allowing the user to choose how many times to run the simulation.  I find that 10,000 repetitions is about adequate.

Check out my SET program here.  It’s still a work in progress.  I’m now deciding whether to calculate the expected value of the number of SETs in a dozen cards, which will mean rewriting some of my methods.

Here’s a hack:  if you forget anything in python you can just google it for a refresher.   For example, I couldn’t remember how to set argument defaults for my functions, so I googled:  “python function set default arguments.”

Summertime is a great time for students to follow rabbit holes and exercise python skills.

 

Factorials –> Factoring

I was working through this year’s MathCounts State Level exam, and found 2 problems involving factorials.  Your reflex when seeing a factorial problem is to see how you can factor the expression.  For example we are asked to find the value of this expression:

\frac{5!+6!}{4!+3!}

Notice that the 2 terms in the numerator are both products of 5! and the 2 terms in the denominator are both products of 3!.  So we can factor these both out:

\frac{5!+6!}{4!+3!}=\frac{5!(1+6)}{3!(4+1)}

If you notice that 5! and 3! both have a factor of 3! then the fraction is much easier to evaluate.

Another problem on the same exam asks us to solve for n:

(n+1)! - n! = 4320

Again you can factor n!  from the 2 terms on the left hand side:

(n+1)! - n! = n!(n+1 - 1)= 4320

n!\cdot n = 4320

To finish out this problem we can find the prime factorization of 4320 and match that up to the product of consecutive integers.

4320 = 2^5 \cdot 3^3 \cdot 5 = 2 \cdot 3 \cdot 2^2 \cdot 5 \cdot (2 \cdot 3) \cdot (2 \cdot 3) = 6! \cdot 6

n = 6

 

Try it and see if it works

I was tutoring a student who is using the AoPS Prealgebra textbook, and we were learning how to solve linear equations with one variable (p. 215).  This is the sort of problem that looks like this:
21n + 28 = 10n - 40.
My student understood that in order to solve the equation we first need to combine like terms and isolate the variable, which he successfully did:
11n = -68.
My student also understood that we need our variable n to have a coefficient of 1 so that we get something that looks like
n = some number.
But how to turn 11n = into n = ?
It turned out he had a lot of interesting ideas.  “How about we subtract 10n from both sides?” he suggested.
Those of you well-versed in algebraic manipulations can foretell that this will lead you farther away from a solution.  But rather than cut him off and tell him this is wrong, I went along with it.
“Okay, let’s give it a try and see what happens.  Subtracting 10n from both sides…”
11n - 10n = -68 - 10n
So: n = -68 - 10n .
Well, that didn’t work.  It turned out my student had many, many creative approaches to isolating n and solving this problem, none of which got us closer to actually solving the problem.  But rather than cutting him off and prematurely telling him he was wrong, I went along with the playful exploration.  This approach of “try it and see if it works” experimentation is something we want to cultivate in our students, not just in math, but in many academic fields.   Eventually he remembered that we could divide both sides by 11 to solve the problem.   While this sort of discovery approach can be inefficient and tedious, it’s fun to pop into a rabbit hole occasionally.
My hope is that in the future, if he sees another problem like this, he won’t panic, but will instead try to reason it out, just as he did when he learned it for the first time.

Mental Math Shortcuts

Today my MathCounts team practiced their first Countdown Round.  This is the spelling bee style competition in which students compete head-to-head on stage to answer the question before their opponent.  Identifying elegant shortcuts is critical to getting the answer asap.  For example:

ink

(Source: 2016 MathCounts Chapter Countdown Round)

The question asks for the difference between the average time and 10 minutes.  While one could find the average of the 3 times and then subtract from 10 minutes, a more clever approach (using smaller numbers) is to average the 3 differences from the start.  Since 10:13 > 10:00t_1 = -13.  The other time differences are 9 and 22.  The sum is -13+9+22=18 and the average is 6.  Smaller numbers means quicker, more accurate calculations.

Answer the Question that was Asked

When I coach my MathCounts teams, I make sure to remind them to answer the question that is being asked.  That is, after all the calculations have been performed and you have a value for x, reread the question, and make sure x is what the problem is asking for.  Only when you are sure you have answered the question should you write it down (or bubble it in).
For example, today I’m reviewing materials for my team, and I came across this problem:
 “The three-digit integer 5A4 is a multiple of six.  What is the sum of all the possible values for the digit represented by A?”
After some calculations, I found 4 possible values for A: 0, 3, 6, and 9.  My first thought was: “The answer is 4!  There are four possible values of A.”  But then I reread the question, and it asks for the sum of all possible values.  The correct answer is
0+3+6+9 = 18. 
Sometimes you can avoid this error by setting  your variable equal to the value the problem is asking for, so that when you do solve for x, you do not need to perform additional calculations to generate the correct answer.
This is a well-known error, and Richard Rusczyk has written about it in the context of developing good problem solving habits.  Some people may describe this as a “trick question” but I think it’s just a matter of taking care to reread the question and answer the question that is being asked, not the question that you wish had been asked.

Mixed up functions

I recently came across an AIME problem that compared what I call “mixed up functions:”

How many real numbers x satisfy the equation:

\frac{1}{5}(\log_2 x)  =sin(5\pi x)

By mixed up, I mean we have the periodic sine curve equated with the monotonically increasing log function.  Often with logs we can replace them with an exponential function, in this case with base 2, but that looks messy.

The approach I often see with these mis-matched functions is to draw a graph and see where the 2 functions intersect.  But first, I’m going to multiply both sides by 5 because who likes fractions?

\log_2 x  =5 sin(5\pi x)

Ah that’s better.  Notice that the maximum and minimum values of y=5 sin(5\pi x) occur when x=\frac{1}{10} and x=\frac{-1}{10}, and the maximum and minimum values are 5 and -5.  And of course this is a periodic function that is defined for all real numbers.

y=\log_2 x on the other hand is defined only over positive reals, and since it is always increasing from left to right, at some point the value of the function will exceed 5.  Specifically, this will occur for x>32.  Likewise, y=\log_2 x will be less than -5 for x<\frac{1}{32}.

And it’s apparent that in the interval \frac{1}{32}<x<32 the log function is going to intersect the sine function as it goes up and down on its periodic path.  How many times does it intersect?  AIME writers like to catch students on “off by one” errors, so take care to examine what is happening close to x=\frac{1}{32} and x=32 before entering your answer!