Logs of this year

Algebra Level 3

A = log 2016 log 2015 log 3 log 2 2 3 . . . 2015 2016 or B = log 2 log 3 log 2015 log 2016 2016 2015 . . . 3 2 \text{A} = \log_{2016} \log_{2015} \cdots \log_3 \log_2 2^{3^{{.}^{{.}^{{.}^{{2015}^{2016}}}}}} \\ \text{or}\\ \text{B} = \log_2 \log_3 \cdots \log_{2015} \log_{2016} {2016}^{{2015}^{{.}^{{.}^{{.}^{{3}^{2}}}}}}

Which of the above expressions is greater?

B \text{B} Both are equal Cannot be determined A \text{A}

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1 solution

Ashish Menon
Jun 25, 2016

log n log n 1 log 3 log 2 2 3 . . . n 1 n = log n log n 1 log 4 log 3 3 4 . . . n 1 n log 2 2 = log n log n 1 log 5 log 4 4 5 . . . n 1 n log 3 3 = log n log n 1 log 6 log 5 5 6 . . . n 1 n log 4 4 = log n log n 1 ( n 1 ) n = log n n log n 1 n 1 = 1 \begin{aligned} \log_{n} \log_{n-1} \cdots \log_3 \log_2 2^{3^{{.}^{{.}^{{.}^{{n-1}^{n}}}}}} & = \log_{n} \log_{n-1} \cdots \log_4 \log_3 3^{4^{{.}^{{.}^{{.}^{{n-1}^{n}}}}}} \log_2 2\\ \\ & = \log_{n} \log_{n-1} \cdots \log_5\log_4 4^{5^{{.}^{{.}^{{.}^{{n-1}^{n}}}}}} \log_3 3\\ \\ & = \log_{n} \log_{n-1} \cdots \log_6 \log_5 5^{6^{{.}^{{.}^{{.}^{{n-1}^{n}}}}}} \log_4 4\\ \\ & \vdots\\ \\ & = \log_{n} \log_{n-1} {(n-1)}^{n}\\ \\ & = \log_{n} n \log_{n-1} n-1\\ \\ & = \color{#20A900}{1} \end{aligned}

log 2 log 3 log n 1 log n n n 1 . . . 3 2 = log 2 log 3 log n 2 log n 1 n 1 n 2 . . . 3 2 log n n = log 2 log 3 log n 3 log n 2 n 2 n 3 . . . 3 2 log n 1 n 1 = log 2 log 3 log n 4 log n 3 n 3 n 4 . . . 3 2 log n 2 n 2 = log 2 log 3 3 2 = log 2 2 log 3 3 = 1 \begin{aligned} \log_2 \log_3 \cdots \log_{n-1} \log_{n} {n}^{{n-1}^{{.}^{{.}^{{.}^{{3}^{2}}}}}} & = \log_2 \log_3 \cdots \log_{n-2} \log_{n-1} {n-1}^{{n-2}^{{.}^{{.}^{{.}^{{3}^{2}}}}}} \log_{n} n\\ \\ & = \log_2 \log_3 \cdots \log_{n-3} \log_{n-2} {n-2}^{{n-3}^{{.}^{{.}^{{.}^{{3}^{2}}}}}} \log_{n-1} n-1\\ \\ & = \log_2 \log_3 \cdots \log_{n-4} \log_{n-3} {n-3}^{{n-4}^{{.}^{{.}^{{.}^{{3}^{2}}}}}} \log_{n-2} n-2\\ \\ & \vdots\\ \\ & = \log_2 \log_3 3^2\\ \\ & = \log_2 2 \log_3 3\\ \\ & = \color{#20A900}{1} \end{aligned}

So, even if we substitute n = 2016 n = 2016 , Both are equal \color{#3D99F6}{\boxed{\text{Both are equal}}} .

I solved it in 2 seconds !!!

abc xyz - 4 years, 11 months ago

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:P Yeah, it happens with those who have practised log a lot ;)

Ashish Menon - 4 years, 11 months ago

Nice problem

Hana Wehbi - 4 years, 11 months ago

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Thanks!! :) :)

Ashish Menon - 4 years, 11 months ago

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