Answer
(a) division
Work Step by Step
When dividing two complex numbers
$(a+bi)\displaystyle \div(c+di)=\frac{a+bi}{c+di}$,
we rationalize the denominator by multiplying with $\displaystyle \frac{c-di}{c-di}$.
The conjugate of $c+di$ is involved.
For the other operations,
$(a+bi)+(c+di)=(a+c)+(b+d)i$,
$(a+bi)-(c+di)=(a-c)+(b-d)i$,
$(a+bi)\cdot(c+di)=(ac-bd)+(ad+bc)i$,
no conjugate is involved.
