answer
-625
foil
first foil (20 + 15i)(15i - 20) to get
(20 * 15i) + (20 * (-20)) + (15i * 15i) + (15i * (-20))
= (300i) + (-400) + (225[tex]i^{2}[/tex]) + (-300i)
combine like terms
combine the i terms together to get
(300i) + (-400) + (225[tex]i^{2}[/tex]) + (-300i)
300i - 400 + 225[tex]i^{2}[/tex] - 300i
= -400 + 225[tex]i^{2}[/tex]
substitute -1 into i^2
remember that i = [tex]\sqrt{-1}[/tex] so [tex]i^{2}[/tex] = -1, and substitute this into -400 + 225i^2
-400 + 225i^2
= -400 + 225(-1)
= -400 -225
= -625
