Is that too big to calculate?

Algebra Level 2

Simplify the following expression:

1 log 2 ( 2019 ! ) + 1 log 3 ( 2019 ! ) + 1 log 4 ( 2019 ! ) + + 1 log 2019 ( 2019 ! ) \frac{1}{\log_2(2019!)}+ \frac{1}{\log_3(2019!)}+\frac{1}{\log_4(2019!)}+\dots+\frac{1}{\log_{2019}(2019!)}


The answer is 1.

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2 solutions

Chew-Seong Cheong
May 13, 2019

S = 1 log 2 2019 ! + 1 log 3 2019 ! + 1 log 4 2019 ! + + 1 log 2019 2019 ! By log b a = log a log b = log 2 log 2019 ! + log 3 log 2019 ! + log 4 log 2019 ! + + log 2019 log 2019 ! = log 2 + log 3 + log 4 + + log 2019 log 2019 ! As log a + log b + log c + + log z = log ( a b c z ) = log 2019 ! log 2019 ! = 1 \begin{aligned} S & = \frac 1{\log_2 2019!} + \frac 1{\log_3 2019!} + \frac 1{\log_4 2019!} + \cdots + \frac 1{\log_{2019} 2019!} & \small \color{#3D99F6} \text{By }\log_b a = \frac {\log a}{\log b} \\ & = \frac {\log 2}{\log 2019!} + \frac {\log 3}{\log 2019!} + \frac {\log 4}{\log 2019!} + \cdots + \frac {\log 2019}{\log 2019!} \\ & = \frac {\log 2 + \log 3 + \log 4 + \cdots + \log 2019}{\log 2019!} & \small \color{#3D99F6} \text{As }\log a + \log b + \log c + \cdots + \log z = \log (abc\cdots z) \\ & = \frac {\log 2019!}{\log 2019!} = \boxed 1 \end{aligned}

Chris Lewis
May 12, 2019

Each of the terms of the sum can be rewritten using 1 log b 2019 ! = log b log 2019 ! \frac{1}{\log_b 2019!}=\frac{\log b}{\log 2019!}

Hence the sum is log 2 + log 3 + + log 2019 log 2019 ! = log 2019 ! log 2019 ! = 1 \frac{\log2+\log3+\cdots +\log{2019}}{\log 2019!}=\frac{\log 2019!}{\log 2019!}=\boxed1

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