Weird Logarithms

Algebra Level 3

2 log 70 9800 × 5 log 70 140 × 7 log 70 2 = ? \large { 2 }^{ \log _{ 70 }{ 9800 } }\times { 5 }^{ \log _{ 70 }{ 140 } }\times { 7 }^{ \log _{ 70 }{ 2 } } = \ ?


The answer is 40.

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2 solutions

Chew-Seong Cheong
Mar 18, 2018

N = 2 log 70 9800 × 5 log 70 140 × 7 log 70 2 = 2 log 70 7 0 2 × 2 × 5 log 70 70 × 2 × 7 log 70 2 = 2 2 + log 70 2 × 5 1 + log 70 2 × 7 log 70 2 = 4 × 2 log 70 2 × 5 × 5 log 70 2 × 7 log 70 2 = 4 × 5 × ( 2 × 5 × 7 ) log 70 2 = 20 × 7 0 log 70 2 = 20 × 2 = 40 \begin{aligned} N & = 2^{\log_{70}9800} \times 5^{\log_{70}140} \times 7^{\log_{70}2} \\ & = 2^{\log_{70}70^2\times 2} \times 5^{\log_{70}70\times 2} \times 7^{\log_{70}2} \\ & = 2^{2+\log_{70}2} \times 5^{1+\log_{70}2} \times 7^{\log_{70}2} \\ & = 4 \times 2^{\log_{70}2} \times 5\times 5^{\log_{70}2} \times 7^{\log_{70}2} \\ & = 4 \times 5 \times (2\times 5 \times 7)^{\log_{70}2} \\ & = 20 \times 70^{\log_{70}2} \\ & = 20 \times 2 \\ & = \boxed{40} \end{aligned}

@Ayush Mishra , I redo the phrasing and LaTex of the problem for you.

Chew-Seong Cheong - 3 years, 2 months ago
Zee Ell
Mar 17, 2018

N = 2 log 70 9800 × 5 log 70 140 × 7 log 70 2 = 2 log 70 7 0 2 × 2 × 5 log 70 70 × 2 × 7 log 70 2 = N = 2 ^ { \log_{70}9800 } × 5 ^ { \log_{70}140 } × 7 ^ { \log_{70}2 } = 2 ^ { \log_{70}70^2 ×2} × 5 ^ { \log_{70}70×2 } × 7 ^ { \log_{70}2 } =

= 2 2 + log 70 2 × 5 1 + log 70 2 × 7 log 70 2 = 2 2 × 5 × 2 log 70 2 × 5 log 70 2 × 7 log 70 2 = = 2 ^ { 2+ \log_{70}2 } × 5 ^ { 1+ \log_{70}2 } × 7 ^ { \log_{70}2 } = 2^2 × 5 × 2 ^ { \log_{70}2 } × 5 ^ { \log_{70}2 } × 7 ^ { \log_{70}2 } =

= 20 × ( 2 × 5 × 7 ) log 70 2 = 20 × 7 0 log 70 2 = 20 × 2 = 40 = 20 × (2 × 5 × 7)^ { \log_{70}2 } = 20 × 70^ { \log_{70}2 } = 20 × 2 = \boxed { 40 }

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