The imaginary unit with the golden ratio!

Algebra Level 4

For i = 1 i = \sqrt{-1} and ϕ = 1 + 5 2 \phi = \dfrac{1+\sqrt5}2 , compute 2 sin ( i ln ϕ ) 2\sin(i \ln\phi) .

ϕ \phi i ϕ \frac i\phi i i ϕ + i \phi + i i ϕ i \phi

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3 solutions

We have:

so:

As s i n ( i x ) = i s i n h ( x ) sin(ix)=isinh(x) we can rewrite the expresion as 2 i s i n h ( l n ϕ ) 2isinh(ln\phi) And using the fact that ϕ \phi is solution for x 2 x 1 = 0 x^2-x-1=0 we can find i ( ϕ 1 ϕ ) = i ( ϕ 2 ϕ 1 + ϕ ϕ ) = i i(\phi -\frac{1}{\phi})=i(\frac{\phi^2 -\phi -1 +\phi}{\phi})=i

Jack Rawlin
Feb 13, 2016

2 sin ( i ln ϕ ) = 2 i sinh ( ln ϕ ) 2\sin (i \ln\phi) = 2i\sinh (\ln\phi)

2 i ( e ln ϕ e ln ϕ 2 ) 2i\left(\frac{e^{\ln\phi} - e^{-\ln\phi}}{2}\right)

2 i ( ϕ 1 ϕ 2 ) 2i\left(\frac{\phi - \frac{1}{\phi}}{2}\right)

i ( ϕ 1 ϕ ) i\left(\phi - \frac{1}{\phi}\right)

i ( 1 + 5 2 2 1 + 5 ) i\left(\frac{1 + \sqrt{5}}{2} - \frac{2}{1 + \sqrt{5}}\right)

i ( 1 + 5 2 2 ( 1 5 ) 4 ) i\left(\frac{1 + \sqrt{5}}{2} - \frac{2(1 - \sqrt{5})}{-4}\right)

i ( 1 + 5 2 + 1 5 2 ) i\left(\frac{1 + \sqrt{5}}{2} + \frac{1 - \sqrt{5}}{2}\right)

i ( 2 2 ) i\left(\frac{2}{2}\right)

i i

2 sin ( i ln ϕ ) = i \large \boxed{2\sin (i \ln\phi) = i}

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