russian log problem

Algebra Level 2

Решите уравнения (solve the equation)


The answer is 125.

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2 solutions

Satyajit Ghosh
Aug 15, 2014

log 5 x + log 5 x + log 1 5 x = 6 log 5 x + 1 1 2 log 5 x + 1 1 log 5 x = 6 log 5 x + 2 log 5 x 1 log 5 x = 6 log 5 x × x 2 x = 6 log 5 x 2 = 6 x = 5 3 x = 125 \log _{ 5 }{ x+\log _{ \sqrt { 5 } }{ x } +\log _{ \frac { 1 }{ 5 } }{ x } =6 } \\ \log _{ 5 }{ x } +\frac { 1 }{ \frac { 1 }{ 2 } } \log _{ 5 }{ x } +\frac { 1 }{ -1 } \log _{ 5 }{ x } =6\\ \log _{ 5 }{ x } +2\log _{ 5 }{ x } -1\log _{ 5 }{ x } =6\\ \log _{ 5 }{ \frac { x\times { x }^{ 2 } }{ x } } =6\\ \log _{ 5 }{ { x }^{ 2 } } =6\\ x={ 5 }^{ 3 }\\ x=125

Bill Bell
Nov 3, 2014

Put all items in terms of base 5 logariths: log 5 x = log 5 x 2 \log _{ \sqrt { 5 } }{ x } =\log _{ 5 }{ { x }^{ 2 } } and log 1 5 x = log 5 5 x \log _{ \frac { 1 }{ 5 } }{ x } =\log _{ 5 }{ { 5 }^{ -x } } . Then the left-hand side becomes 5 x x 2 5 x { 5 }^{ x }{ x }^{ 2 }{ 5 }^{ -x } and the right 5 6 { 5 }^{ 6 } . Cancelling and solving for x x we obtain x = 125 x=125 .

Nice shortest method...

Mohd Mazhar - 3 years, 6 months ago

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