Converting Complex numbers into Cartesian Form

I am trying to convert the following Complex number equation into Cartesian Form:

$$\sqrt{8}\left(\cos \frac{\pi}{4} + i \sin\frac{\pi}{4}\right)$$

So far I have tried doing both:

$\sqrt{8} (\pi/4)\cos (\pi/4)$ and $\sqrt{8}(\pi/4)\sin (\pi/4)$ which returns $(0.5\pi, 0.5\pi)$ but the tutorial I am following says this is the incorrect answer.. Does anyone know how to correctly convert Complex numbers in Polar form to Cartesian Form?

• How did you go from $\sqrt8\cos\frac\pi4$ to $\sqrt8\frac\pi4\cos\frac\pi4$? – Lord Shark the Unknown May 14 '17 at 13:06
• You may not like it, but this is the Cartesian form (i.e., $z=x+iy$). The polar form (as suggested below) is $z=re^{i\theta}=\sqrt{8}e^{i\pi/4}$. – Cye Waldman Jul 30 '17 at 20:45

A direct relation between the cartesian and polar representation a complex number is provided by Euler's formula

$$re^{i\theta} = r\cos\theta + i\sin\theta$$

You should compare this agains the number you have, which allows to conclude that

$$r = \sqrt{8} ~~~\mbox{and}~~~ \theta = \pi/4$$

that is

$$\sqrt{8}\left(\cos \frac{\pi}{4} + i \sin\frac{\pi}{4}\right) = \sqrt{8}e^{i\pi/4}$$

It's as simple as multiplying it out.

$\sin \frac{\pi}{4} =\cos \frac{\pi}{4} = \frac 1{\sqrt 2}$ so $\cos \frac{\pi}{4} + i \sin \frac{\pi}{4} = \frac 1{\sqrt 2}(1+i)$

So $\sqrt 8 (\cos \frac{\pi}{4} + i \sin \frac{\pi}{4} ) = (2\sqrt 2)(\frac 1{\sqrt 2})(1+i) = 2(1+i) = 2 + 2i$

• So simple, thanks for pointing it out. If the equation was changed to $(cos - pi/4 + i sin - pi/4)$ would there be a different method for that or simply calculate $(cos(-pi/4)$ and then use the method above? – bancqm May 14 '17 at 14:16
• Just work out the respective trig ratios the same way. So that would be $2-2i$ – Deepak May 14 '17 at 23:26