(x-1)(x+1)(x+3)(x+5)-20
=[(x-1)(x+5)][(x+1)(x+3)]-20
=(x²+4x-5)(x²+4x+3)-24
=(x²+4x)²-5(x²+4x)+3(x²+4x)-15-20
=(x²+4x)²-2(x²+4x)-35
=(x²+4x+5)(x²+4x-7)
(x-1)(x+1)(x+3)(x+5)-20
=[(x-1)(x+5)][(x+1)(x+3)]-20
=(x²+4x-5)(x²+4x+3)-24
=(x²+4x)²-5(x²+4x)+3(x²+4x)-15-20
=(x²+4x)²-2(x²+4x)-35
=(x²+4x+5)(x²+4x-7)