1.
(-x-1)²×(x-1)²-(x+3)²×(x-3)²
=[(x+1)(x-1)]²-[(x+3)(x-3)]²
=(x²-1)²-(x²-9)²
=x^4-2x²+1-x^4+18x-81
=16x-80
=16(x-5)
2.
(xy+1)²-(x+y)²
=(xy+x+y+1)(xy-x-y+1)
=(x+1)(y+1)(x-1)(y-1)
3.
(x²+2x)²-2(x²+2x)-3
=(x²+2x)²+(x²+2x)-3(x²+2x)-3
=(x²+2x)(x²+2x+1)-3(x²+2x+1)
=(x²+2x+1)(x²+2x-3)
5.
5x²-14xy+8y²
=5x²-4xy-10xy+8y²
=x(5x-4y)-2y(5x-4y)
=(5x-4y)(x-2y)