a)(5x−2y)(x2−xy+1)
=5x(x2−xy+1)−2y(x2−xy+1)
=5x3−5x2y+5x−2x2y+2yx2y−2y
=5x3−7x2y+5x+2xy2−2y
b) (x−1)(x+1)(x+2)
=[x(x+1)−1(x+1)](x+2)
=(2x+x−x−1)(x+2)
=(x2−1)(x+2)
=x2(x+2)−1(x+2)
=x3+2x2−x−2
c) 12x2y2(2x+y)(2x−y)
=12x2y2[2x(2x−y)+y(2x−y)]
=12x2y2(4x2−2y+2y− y2)
=12x2y2(4x2− y2)
=12x2y2.4x−12x2y2.(− y2)
=14x4y2−12 x2y2