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Difference between revisions of "2013 AIME II Problems/Problem 2"

(Problem 2)
 
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<cmath>\log_2(\log_{2^a}(\log_{2^b}(2^{1000}))) = 0.</cmath>
 
<cmath>\log_2(\log_{2^a}(\log_{2^b}(2^{1000}))) = 0.</cmath>
 
Find the sum of all possible values of <math>a+b</math>.
 
Find the sum of all possible values of <math>a+b</math>.
 +
 +
==Solution==
 +
To simplify, we write this logarithmic expression as an exponential one. Just looking at  the first log, it has a base of 2 and an argument of the expression in parenthesis. Therefore, we can make 2 the base, 0 the exponent, and the arguement the result. That means <math>\log_{2^a}(\log_{2^b}(2^{1000}))=1</math> (Because <math>2^0=1</math>). Doing this again, we get <math>\log_{2^b}(2^{1000})=2^a</math>. Doing the process one more time, we finally eliminate all of the logs, getting <math>{2^{b}}^{2^a}=2^{1000}</math>. Using the property that <math>{a^{x^{y}}=a^{xy}</math>, we simplify to <math>2^{b*2^{a}}=2^{1000}</math>. Eliminating equal bases leaves <math>b*2^a=1000</math>. The largest a such that <math>2^a</math> divides <math>1000</math> is <math>3</math>, so we only need to check <math>1</math>,<math>2</math>, and <math>3</math>. When <math>a=1</math>, <math>b=500</math>; when <math>a=2</math>, <math>b=250</math>; when <math>a=3</math>, <math>b=125</math>. Summing all the a's and b's gives the answer of <math>\boxed{881}</math>

Revision as of 17:31, 4 April 2013

Positive integers $a$ and $b$ satisfy the condition \[\log_2(\log_{2^a}(\log_{2^b}(2^{1000}))) = 0.\] Find the sum of all possible values of $a+b$.

Solution

To simplify, we write this logarithmic expression as an exponential one. Just looking at the first log, it has a base of 2 and an argument of the expression in parenthesis. Therefore, we can make 2 the base, 0 the exponent, and the arguement the result. That means $\log_{2^a}(\log_{2^b}(2^{1000}))=1$ (Because $2^0=1$). Doing this again, we get $\log_{2^b}(2^{1000})=2^a$. Doing the process one more time, we finally eliminate all of the logs, getting ${2^{b}}^{2^a}=2^{1000}$. Using the property that ${a^{x^{y}}=a^{xy}$ (Error compiling LaTeX. Unknown error_msg), we simplify to $2^{b*2^{a}}=2^{1000}$. Eliminating equal bases leaves $b*2^a=1000$. The largest a such that $2^a$ divides $1000$ is $3$, so we only need to check $1$,$2$, and $3$. When $a=1$, $b=500$; when $a=2$, $b=250$; when $a=3$, $b=125$. Summing all the a's and b's gives the answer of $\boxed{881}$

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