Is this trig even real?

Algebra Level 3

Find the principle value of z z for the equation:

sin ( z ) = 2 \sin(z)=2

e i ln ( π ) e^{i\ln(\pi)} i i i π 2 \frac{i\pi}{\sqrt{2}} ln ( i π ) e i π 4 \ln(i\pi)-e^{i\frac{\pi}{4}} Undefined π 2 ln ( 2 ± 3 ) i \frac{\pi}{2}-\ln(2 \pm \sqrt{3})i

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1 solution

James Watson
Jun 17, 2020

For this, we need to use the complex definition of the sine function (derived from Euler's Formula ):

sin ( z ) = e i z e i z 2 i \sin(z)=\frac{e^{iz}-e^{-iz}}{2i}

We can set this to be 2 2 and then we can neaten it up:

e i z e i z 2 i = 2 e i z e i z = 4 i \frac{e^{iz}-e^{-iz}}{2i}=2 \Longrightarrow e^{iz}-e^{-iz}=4i

We can now multiply both sides by e i z e^{iz} and we can form a quadratic equation:

e i z e i z = 4 i ( e i z ) 2 1 = 4 i e i z ( e i z ) 2 4 i e i z 1 = 0 e^{iz}-e^{-iz}=4i \Longrightarrow \left(e^{iz}\right)^2-1=4ie^{iz} \Longrightarrow \left(e^{iz}\right)^2-4ie^{iz}-1=0

Now, using the Quadratic Formula , we can determine that, with a = 1 , b = 4 i , c = 1 a=1, b=-4i, c=-1 :

e i z = ( 4 i ) ± ( 4 i ) 2 4 ( 1 ) ( 1 ) 2 ( 1 ) e i z = 4 i ± 12 2 e i z = i ( 2 ± 3 ) e^{iz}=\frac{-(-4i)\pm\sqrt{(-4i)^2-4(1)(-1)}}{2(1)} \Longrightarrow e^{iz}=\frac{4i\pm\sqrt{-12}}{2} \Longrightarrow e^{iz}=i(2\pm\sqrt{3})

Now, we can take the natural log to get:

i z = ln ( i ( 2 ± 3 ) i z = ln ( i ) + ln ( 2 ± 3 ) iz=\ln(i(2\pm\sqrt{3}) \Longrightarrow iz=\ln(i)+\ln(2\pm\sqrt{3})

Since we know that ln ( i ) = e i π 2 \ln(i)=e^{i\frac{\pi}{2}} and we are only taking the principle value we can say that

i z = i π 2 + ln ( 2 ± 3 ) iz=i\frac{\pi}{2}+\ln(2\pm\sqrt{3})

Now we can divide by i i to find that

z = π 2 ln ( 2 ± 3 ) i \boxed{z=\frac{\pi}{2}-\ln(2\pm\sqrt{3})i}

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