xy²-2xy+2y-4
=xy(y-2)+2(y-2)
=(xy+2)(y-2)
9x²-y²-4y-4
=9x^2-(y^2+4y+4)
=9x^2-(y+2)^2
=(3x+y+2)(3x-y-2)
4xy²-4x²y-y³
=-y(y^2-4xy+4x^2)
=-y(2x-y)^2
- a²-1+b²-2a
= b²-a²-1-2a
=b^2-(a^2+2a+1)
=b^2-(a+1)^2
=(b+a+1)(b-a-1)
(99又¼)×(100又¾)
=(100-3/4)(100+3/4)
=100^2-(3/4)^2
=10000-9/16
=9999+7/16
=9999又7/16
100²-99²+98²-97²+…+4²-3²+2²-1²
=(100-99)(100+99)+(98-97)(98+97)+.+(4-3)(4+3)+(2-1)(2+1)
=100+99+98+97+...+4+3+2+1
=(1+100)*100/2
=5050
（1-2的平方分之一）（1-3的平方分之一）（1-4的平方分之一）…（1-10的平方分之一）=
=(1-1/2)(1+1/2)(1-1/3)1+1/3)(1-1/4)(1+1/4).(1-1/10)(1+1/10)
=1/2*3/2*2/3*4/3*3/4*5/4*.*9/10*11/10
=1/2*11/10
=11/20