1.4分之1的x的2次方减25y的平方 等于?
1/4x^2-25y^2
=[1/2x+5y][1/2x-5y]

2.（x+y)的平方减去z平方
(x+y)^2-z^2
=[x+y+z][x+y-z]
3.2ab-a^2-b^2
=-[a^2-2ab+b^2]
=-[a-b]^2
4.x^2*y^2+1-2xy
=[xy-1]^2