Question

The complex number z = –1 + 3i can be expressed in trigonometric form as:

Answer 1

$\bf z=\stackrel{a}{-1}\stackrel{b}{+3}i\qquad \begin{cases} r=\sqrt{a^2+b^2}\\ \theta =tan^{-1}\left( \frac{b}{a} \right) \end{cases}\qquad \qquad z=r[cos(\theta )i~sin(\theta )]\\\\ -------------------------------\\\\ r=\sqrt{(-1)^2+3^2}\implies r=\sqrt{10} \\\\\\ \theta =tan^{-1}\left( \frac{3}{-1} \right)\implies \theta \approx -71.57\implies \theta \approx\stackrel{360-71.57}{288.43^o}\\\\ -------------------------------\\\\ z=\sqrt{10}\left[ cos(288.43^o)+i~sin(288.43^o) \right]$

now, the calculation in the choices, show 289.5°, which I take it is due some early rounding up.