Answer
$i^{3}=-i$.
Work Step by Step
$i=\sqrt -1$ hence $i^{2}=\sqrt -1\times\sqrt -1=-1$ and $i^{3}=i\times i^{2}=\sqrt -1\times -1=-\sqrt -1=-i$.
$a+bi$ where $a$ is the real part and $b$ is the imaginary part hance for this answer $a=0$ as there is no real part and $b=-1$ giving $-i$ as the answer.
