a) Ta có (1 + 2y)2 + (1 - 2y)2 + 2(1 + 2y)(1 - 2y)
= (1 + 2y + 1 - 2y)2 = 22 = 4
b) Ta có (7x + 2y)2 + (7x - 2y)2 - 2(49x2 - 4y2)
= (7x + 2y)2 + (7x - 2y)2 - 2[(7x)2 - (2y)2]
= (7x + 2y)2 + (7x - 2y)2 - 2(7x - 2y)(7x + 2y)
= (7x + 2y - 7x + 2y)2
= (4y)2 = 16y2