AIME Logarithm problem!

Level 2

Suppose that a , b , c a,b,c are real positive integers such that, a log 3 7 = 27 , b log 7 11 = 49 , c log 11 25 = 11 a^{\log_{3}7}=27,b^{\log_{7}11}=49,c^{\log_{11}25}=\sqrt{11} .Find the value of, a ( log 3 7 ) 2 + b ( log 7 11 ) 2 + c ( log 11 25 ) 2 a^{(\log_{3}7)^{2}}+b^{(\log_{7}11)^2}+c^{(\log_{11}25)^2} .


The answer is 469.

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1 solution

Brian Shen
Jun 16, 2020

This problem is not based around solving individual a a , b b , and c c . Instead, it’s more about manipulating the values given to get the desired outcome.

Let’s look at the first value in the final outcome we’re supposed to find: a ( log 3 7 ) 2 a^{(\log_3 7)^2} . Perhaps if we notice something with the first value, we can apply it to the others since they are all relatively similar.

Substituting in a ( log 3 7 ) = 27 a^{(\log_3 7)} = 27 , we get 2 7 log 3 7 27^{\log_3 7} . Recall that if we have a log a b a^{\log_a b} , it is equivalent to b b . Here, we can express 27 27 as 3 3 3^3 , so the resulting expression is 3 3 log 3 7 3^{3\log_3 7} , or 3 log 3 343 3^{\log_3 343} , which simplifies to 343 343 . Aha! Now we have found a method that we can apply similarly to all of the other values.

Applying this method to b b , we get that b ( log 7 11 ) 2 b^{(\log_7 11)^2} is equivalent to 4 9 log 7 11 49^{\log_7 11} which simplifies to 7 2 log 7 11 7^{2\log_7 11} , where we end up with 121 121 .

c c is a similar process as well, where c ( log 11 25 ) 2 c^{(\log_{11} 25)^2} simplifies down to 11 log 11 25 \sqrt{11}^{\log_{11} 25} , which is 1 1 ( 1 2 log 11 25 ) 11^{(\frac{1}{2}\log_{11} 25)} , which simplifies to 5 5

343 + 121 + 5 343 + 121 + 5 gives us our final answer of 469 \boxed{469}

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