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:
Notice that the 2 terms in the numerator are both products of and the 2 terms in the denominator are both products of . So we can factor these both out:
If you notice that and both have a factor of then the fraction is much easier to evaluate.
Another problem on the same exam asks us to solve for :
Again you can factor from the 2 terms on the left hand side:
To finish out this problem we can find the prime factorization of and match that up to the product of consecutive integers.
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:
(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 , . The other time differences are and . The sum is and the average is Smaller numbers means quicker, more accurate calculations.
I recently came across an AIME problem that compared what I call “mixed up functions:”
How many real numbers x satisfy the equation:
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?
Ah that’s better. Notice that the maximum and minimum values of occur when and , 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.
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 . Likewise, will be less than -5 for .
And it’s apparent that in the interval 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 and before entering your answer!
The best way to prepare for any test is to locate old versions of the exam and practice with those, especially studying the problems you answered incorrectly. This is true for the AMCs and MathCounts. AoPS also offers online prep classes. For those of you who prefer learning from textbooks, here are few recommendations. These books take problems from old competitions and organize them by topic so you can ramp up your skills.
Competition Math for Middle School by J. Batterson. In particular I enjoyed the chapters on counting and probability for their concise and clear introductions to the subjects. You’ll be up to speed on these topics in no time. Other chapters are algebra, geometry, and number theory.
First Steps for Math Olympians by J. Douglas Faires. This is the next step in difficulty is preparing for the AMC 10/12. This book uses questions from those exams as exercises. Content includes: ratios, polynomials, functions, triangles, circles, polygons, counting, probability, primes, number theory, sequences and series, statistics, trig, 3D geometry, logs, and complex numbers.
A Gentle Introduction to the American Invitational Mathematics Exam by Scott Annin. This title seemed to be speaking to me. I qualified for AIME my senior year of high school, but bombed the test, so I’ve been living in fear of the AIME every since. I’m working through the chapter 1 and I’m pleased to find myself actually solving some problems without assistance. Contents include: algebra, combinatorics, probability, number theory, sequences/series, logs, trig, complex numbers, polynomials, plane geometry, and 3D geometry. Nearly all exercises are former AIME problems.