To answer complex answer, there is a given formula that:
i=√-1
i²=-1
Lets say for example:
(4+i)²
Do the square of binomial:
(4+i)(4+i)
=16+4i+4i+i²
Add all like terms:
16+8i+i²
Then since i²=-1, substitute the value:
16+8i-1
8i+15
We didn't do the 8i to 8√-1 because remember that there shuld be no negative number in square roots, so.... there:D
Hope this helps =)