# Chapter 8 - Section 8.3 - Polar Form of Complex Numbers; De Moivre's Theorem - 8.3 Exercises: 34

$z=10(\displaystyle \cos\frac{2\pi}{3}+i\sin\frac{2\pi}{3})$

#### Work Step by Step

See p. 604, A complex number $z=a+bi$ has the polar (or trigonometric) form $z=r(\cos\theta+i\sin\theta)$ where $r=|z|=\sqrt{a^{2}+b^{2}}$ and $\tan\theta=b/a$. The number $r$ is the modulus of $z$, and $\theta$ is an argument of $z$. ------- $z=-5+5\sqrt{3}i$ $r=|z|=\sqrt{(-5)^{2}+(5\sqrt{3})^{2}}=\sqrt{25+25\cdot 3}=10$ $\displaystyle \tan\theta=\frac{5\sqrt{3}}{-5}=-\sqrt{3}$, In quadrant I, $\displaystyle \tan\frac{\pi}{3}=\sqrt{3}$. By symmetry (on the unit circle), $\theta$ can be $\displaystyle \frac{2\pi}{3}$ (quadrant II) or $\displaystyle \frac{5\pi}{3}$ (quadrant IV) $z=-5+5\sqrt{3}i$ lies in quadrant II, so we select the appropriate argument: $z=10(\displaystyle \cos\frac{2\pi}{3}+i\sin\frac{2\pi}{3})$

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