Difference between revisions of "2008 AMC 10A Problems/Problem 7"

m (Solution)
(Solution)
Line 7: Line 7:
  
 
==Solution==
 
==Solution==
Notice that <math>9</math> can be factored out of the numerator:
+
Simplifying, we get <cmath>\frac{3^4016-3^4012}{3^4014-3^4010}.</cmath> factoring out <math>3^4012</math> on the top and factoring out <math>3^4010</math> on the bottom gives us <cmath>\frac{(3^4-1)(3^4012)}{(3^4-1)(3^4010)}.</cmath> Canceling out <math>3^4-1</math> gives us <math>\frac{3^4012}{3^4010}=\frac{3^2}{3^0}=9.</math>
<cmath>\frac{\left(3^{2008}\right)^2-\left(3^{2006}\right)^2}{\left(3^{2007}\right)^2-\left(3^{2005}\right)^2}=\frac{9\left(3^{2007}\right)^2-9\left(3^{2005}\right)^2}{\left(3^{2007}\right)^2-\left(3^{2005}\right)^2}=9\cdot\frac{\left(3^{2007}\right)^2-\left(3^{2005}\right)^2}{\left(3^{2007}\right)^2-\left(3^{2005}\right)^2}</cmath>
 
Thus, the expression is equal to <math>9</math>, and the answer is <math>\mathrm{(E)}</math>.
 
  
 
==See also==
 
==See also==
 
{{AMC10 box|year=2008|ab=A|num-b=6|num-a=8}}
 
{{AMC10 box|year=2008|ab=A|num-b=6|num-a=8}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 19:17, 30 November 2019

Problem

The fraction \[\frac{\left(3^{2008}\right)^2-\left(3^{2006}\right)^2}{\left(3^{2007}\right)^2-\left(3^{2005}\right)^2}\] simplifies to which of the following?

$\mathrm{(A)}\ 1\qquad\mathrm{(B)}\ \frac{9}{4}\qquad\mathrm{(C)}\ 3\qquad\mathrm{(D)}\ \frac{9}{2}\qquad\mathrm{(E)}\ 9$

Solution

Simplifying, we get \[\frac{3^4016-3^4012}{3^4014-3^4010}.\] factoring out $3^4012$ on the top and factoring out $3^4010$ on the bottom gives us \[\frac{(3^4-1)(3^4012)}{(3^4-1)(3^4010)}.\] Canceling out $3^4-1$ gives us $\frac{3^4012}{3^4010}=\frac{3^2}{3^0}=9.$

See also

2008 AMC 10A (ProblemsAnswer KeyResources)
Preceded by
Problem 6
Followed by
Problem 8
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 10 Problems and Solutions

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. AMC logo.png