A,
x^2 - y^2 -2x -2y
= (x^2 - y^2) -(2x +2y)
= (x+y)(x-y) -2(x+y)
= (x+y)(x-y-2)
B,
5x^6 - 320
=5(x^6 - 64)
=5( (x^3)^2 - 8^2)
= 5( x^3 - 8)(x^3+8)
=5(x-2)(x^2 + 2x+4)(x+2)(x^2-2x-4)
Đặt x2 + 4x - 3 = t ,thì ta có : t2 - 5xt + 6x2
= t2 - 2xt - 3xt + 6x2
= t ( t - 2x ) - 3x ( t - 2x )
= ( t - 2x )( t - 3x )
Thay t = x2 + 4x - 3 , ta có : ( x2 +4x -3 -2x ) - ( x2 + 4x -3 - 3x )
= ( x2 + 2x - 3)( x2 + x -3 )