Series of logarithms

Algebra Level 2

If l o g 3 a + l o g 3 1 2 a + l o g 3 1 3 a + l o g 3 1 4 a . . . . . . . . l o g 3 1 10 a = 110 log_{3}a + log_{3^{\frac{1}{2}}}a + log_{3^{\frac{1}{3}}}a + log_{3^{\frac{1}{4}}}a ........ log_{3^{\frac{1}{10}}}a = 110 , then the value of a a is


The answer is 9.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

We can write the given equation as:

log a log 3 + log a 1 2 log 3 + log a 1 3 log 3 + log a 1 4 log 3 + + log a 1 10 log 3 = 110 \frac{\log a}{\log 3} + \frac{\log a}{\frac{1}{2}\log 3} + \frac{\log a}{\frac{1}{3}\log 3} + \frac{\log a}{\frac{1}{4}\log 3} + \ldots + \frac{\log a}{\frac{1}{10}\log 3} = 110

then factor out log a log 3 \frac{\log a}{\log 3}

log a log 3 ( 1 + 1 1 2 + 1 1 3 + 1 1 4 + + 1 1 10 ) = 110 \frac{\log a}{\log 3}(1 + \frac{1}{\frac{1}{2}} + \frac{1}{\frac{1}{3}} + \frac{1}{\frac{1}{4}} + \ldots + \frac{1}{\frac{1}{10}}) = 110

log a log 3 ( 1 + 2 + 3 + 4 + + 10 ) = 110 \Rightarrow \frac{\log a}{\log 3}(1 + 2 + 3 + 4 + \ldots + 10) = 110

log a log 3 ( 55 ) = 110 \Rightarrow \frac{\log a}{\log 3}(55) = 110

log a log 3 = 110 55 = 2 \Rightarrow \frac{\log a}{\log 3} = \frac{110}{55} = 2

log a = 2 log 3 = log 3 2 = log 9 \Rightarrow \log a = 2\log 3 = \log 3^2 = \log 9

which implies a = 9 a = \boxed{9}

did in 3 seconds

Samya Chaudhary - 7 years, 1 month ago

log {a^{p/q}} b = (q/p)(log a b)

Gabriel Lefundes - 7 years, 1 month ago
Noel Lo
May 30, 2015

Hi Shabarish, I enjoyed this problem!!!! :)

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...