a) (1+2i)(3-i) = 3 - i + 6i - 2i² = 3 + 5i +2 = 5 + 5i
b) (2+i)(-1/2 + i) = -1 + 2i -(1/2)i + i²
= -1 + (3/2)i -1
= -2 + (3/2)i
c) (-3i/4)(-i/2 +2) = +3i²/8 - 3i/2 = -(3/8) -(3/2)i
d) (2-i)(4+i)(i-1) = (8+2i-4i - i²)(i-1) = (9 -2i)(i-1) = 9i-9-2i²+2i = -7 + 11i
mais il faut vérifier les calculs, on ne sait jamais
