①4(x+y)²-9(x-y)²=[2(x+y)+3(x-y)][2(x+y)-3(x-y)]=(5x-y)(-x+5y)②(x²+1)²-20(x²+1)+100=(x²+1-10)²=(x²-9)²=(x+3)²(x-3)²;③x²+5x-6=(x+6)(x-1);④...