Answer
(a) $ 3(cos90^\circ+i\ sin90^\circ)=3cis90^\circ$
(b) $ \sqrt 5(cos63.43^\circ+i\ sin63.43^\circ)=\sqrt 5cis63.43^\circ$
(c) $ 2(cos240^\circ+i\ sin240^\circ)=2cis240^\circ$
Work Step by Step
(a) $3i=3(0+i)=3(cos90^\circ+i\ sin90^\circ)=3cis90^\circ$
(b) $1+2i=\sqrt {5}(\frac{\sqrt 5}{5}+\frac{2\sqrt 5}{5}i)=\sqrt 5(cos63.43^\circ+i\ sin63.43^\circ)=\sqrt 5cis63.43^\circ$
(c) $-1-i\sqrt 3=2(-\frac{1}{2}-\frac{\sqrt 3}{2}i)=2(cos240^\circ+i\ sin240^\circ)=2cis240^\circ$
