2013 AMC 10A Problem 8
What is the value of
Show solution without answering
Key insight
Write 2 to the 2014 as 4 times 2 to the 2012, pull 2 to the 2012 out of top and bottom, and 5 over 3 is left.
Solution
The powers are huge, so the actual numbers cannot matter. Only how they compare matters.
1. Try small numbers first. Replace and by and :
Notice is times . The top is and the bottom is .
2. Do the same with the real numbers. Multiplying by adds to the exponent, so
3. Pull out the common part. Both the top and the bottom contain :
4. Cancel. Now is multiplied on top and bottom, so it cancels, leaving .
The answer is .
Why this works
When you add or subtract powers of the same number, pull out the smallest power first. What remains is small and easy. You may only cancel something from top and bottom when it multiplies everything, which is why Step 3 comes before Step 4. Trying a small case (Step 1) is a good way to see the pattern and to check the answer.
Concise solution
A shorter write-up, the way contest solutions are usually written. It may take a different route from the solution above.
Write . Then both the numerator and the denominator have the common factor :
The answer is .
Review in the AoPS books
- Introduction to Algebra, Chapter 1: Follow the Rules (§1.6, §1.7, §1.8)
- Introduction to Algebra, Chapter 11: Special Factorizations (§11.2, §11.3, §11.5)
- Introduction to Algebra, Chapter 22: Special Manipulations
- Introduction to Algebra, Chapter 19: Exponents and Logarithms (§19.4)
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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site.