Find the value of following expression:
i5 + i10 + i15
i5 + i10 + i15 = i(4 + 1) + i(8 + 2) + i(12 + 3)
= (i4)1×i + (i4)2×i2 + (i4)3×i3
= (i4)1×i + (i4)2×i2 + (i4)3×i2×i
= 1×i + 1×(– 1) + 1×(– 1)×i
= i – 1 – i = – 1