(i) (-12)3 + (7)3 + (5)3
Let -12 = x, 7 = y, 5 = z, then
x3 + y3 + z3 = 3xyz.
∴ (-12)3 + (7)3 + (5)3
= 3(-12)(7)(5)
= -36 × 35
= -1260.
(ii) (28)3 + (-15)3 + (-13)3
Let x = 28, y = -15, z = -13, then
x3 + y3 + z3 = 3xyz
∴ (28)3 + (-15)3 + (-13)3
= 3(28)(- 15)(-13)
= 84 × 195
= 16380