Giải:
a) x2 - 2xy + y2 - 25 = (x - y)2 - 52 = (x - y - 5)(x - y + 5)
b) x2 - 10x - y2 + 25 = (x2 - 10x + 25) - y2 = (x - 5)2 - y2 = (x - 5 - y)(x - 5 + y)
c) 3x + 3y + zx + zy = (3x + 3y) + (zx + zy) = 3(x + y) + z(x + y) = (x + y)(3 + z)
d) x2 - 4y2 + 5x +10y = (x2 - 4y2) + (5x +10y) = (x - 2y)(x + 2y) + 5(x + 2y) = (x + 2y)(x - 2y + 5)