Chứng minh đẳng thức sau:
a) (a + b + c) . (a - b + c) = a2 - b2 + c2 + 2ac
b) (3x + 2y) . (3x - 2y) - (4x - 2y) . (4x + 2y) = -7x2
c) (x - 1) . (x2 + x + 1) - (x +1) . (x2 - x +1) = -2
d) (2x + 1) . (4x2 - 2x + 1) - (2x - 1) . (4x2 +2x + 1) = 2
e) (x - 2y) . (x2 + 2xy + 4y2) - (2x - 1) . (x2 + 2xy + 4y2) = -16y3