Golden Ratio In A Logarithmic Equation?

Algebra Level 4

log a b = log b a + 1 \large{\log _{a}{b} =\log _{b}{a} +1}

Solve for a a the logarithmic equation above, assuming that φ \varphi is the golden ratio.

( φ + 1 ) b , ( φ 1 ) b (\varphi+1)b,\;\, (\varphi-1)b 1 b φ 1 , b φ \frac { 1 }{ { b }^{ \varphi -1 } } ,\;\,{ b }^{ \varphi } 1 b φ , b φ 1 \frac { 1 }{ { b }^{ \varphi } } ,\;\,{ b }^{ \varphi -1 } b b Impossible

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1 solution

By the change of base rule, we can write the given equation as

1 log b ( a ) = log b ( a ) + 1 ( log b ( a ) ) 2 + log b ( a ) 1 = 0 \dfrac{1}{\log_{b}(a)} = \log_{b}(a) + 1 \Longrightarrow (\log_{b}(a))^{2} + \log_{b}(a) - 1 = 0 , and so log b ( a ) = 1 ± 5 2 . \log_{b}(a) = \dfrac{-1 \pm \sqrt{5}}{2}.

With ϕ = 5 + 1 2 \phi = \dfrac{\sqrt{5} + 1}{2} this implies that either log b ( a ) = ϕ 1 \log_{b}(a) = \phi - 1 or log b ( a ) = ϕ , \log_{b}(a) = -\phi,

i.e., that either a = b ϕ 1 \boxed{a = b^{\phi - 1}} or a = 1 b ϕ . a = \boxed{\dfrac{1}{b^{\phi}}}.

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