Complex sine

Algebra Level 2

Which of the following is a solution to sin x = 2 \sin{x} = 2 in the complex numbers?

π 2 ± i ln ( 2 + 3 ) \frac\pi2 \pm i \ln\big(2 + \sqrt{3}\big) π 4 ± i ln ( 2 + 3 ) - \frac\pi4 \pm i \ln\big(2 + \sqrt{3}\big) π 4 ± i ln ( 2 + 3 ) \frac\pi4 \pm i \ln\big(2 + \sqrt{3}\big) π 2 ± i ln 3 \frac\pi2 \pm i \ln{3} π 4 ± i ln 3 \frac\pi4 \pm i \ln{3}

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4 solutions

Rishav Koirala
Jul 9, 2016

Why we can't take z = r.cisx = 2i±√3i , which is more general form of a complex number

Nawal Singh - 3 years, 5 months ago
Mountain Cheng
Nov 18, 2019

Here's a really lazy method. In the example we already get the solution for cos x = 2 \cos{x} = 2 , which is x = 2 k π ± ln 2 ± 3 x = 2k\pi \pm \ln{2 \pm \sqrt{3}} and x = 2 k π ln 2 ± 3 x = 2k\pi \mp \ln{2 \pm \sqrt{3}} . Fortunately, of the options provided, the part outside the logarithm has magnitude less than 2 π 2\pi , so we can drop all the 2 k π 2k\pi to get x = ± ln 2 ± 3 x = \pm \ln{2 \pm \sqrt{3}} or x = ln 2 ± 3 x = \mp \ln{2 \pm \sqrt{3}} . Now, I used the complimentary angle identity: sin x = cos π 2 x \sin{x} = \cos{\frac{\pi}{2} - x} , letting sin x = 2 \sin{x} = 2 : π 2 x = ± ln 2 ± 3 \frac{\pi}{2} - x = \pm\mp \ln{2 \pm \sqrt{3}} , so we have x = π 2 ± ln 2 ± 3 x = \frac{\pi}{2} \pm\mp \ln{2 \pm \sqrt{3}} . Check the options and only one match this form.

展豪 張
May 19, 2016

Are the options π 2 ± i ( ln ( 2 + 3 ) ) \dfrac \pi 2 \pm i (\ln (2+\sqrt 3)) and π 2 ± i ( ln ( 2 3 ) ) \dfrac \pi 2 \pm i (\ln (2-\sqrt 3)) identical?

Yes they are same.

Puneet Pinku - 5 years ago

Why have we not considered the cases in which there is 3 π / 2 3 \pi /2 instead of π / 2 \pi/2 . If we do then we get four solutions to the given question. Do you have any idea about this?

Puneet Pinku - 5 years ago

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I don't think that replacing π / 2 \pi/2 by 3 π / 2 3\pi/2 are still solutions. Can you elaborate more on it?

展豪 張 - 5 years ago

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Check the link which I have provided in my answer to get an idea of what I am saying.

Puneet Pinku - 5 years ago
Puneet Pinku
May 17, 2016

The answer gets very complicated difficult for me to type in. A direct link to a website is given which provides the direct formula and its proof for these sin problems.

Can you solve sin x = 2?

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