解：①x²-y²-z²+2yz=x²-(y²+z²-2yz)=x²-(y-z)²=(x+y-z)(x-y+z) ②-x²+x+2=-(x+1)(x-2) ③(x-1)(x-3)(x+1)(x+3)-20=(x²-9)(x²-1)-20=(x²)²-10x²-11=(x²+1)(x²-11)=(x²+1)(x+√11)(x-√11)
