1/4x^2-y^2 =（1/2x+y)(1/2x-y) (2x+1)^2-(x-2)^2 =(2x+1-x+2)(2x+1+x-2) =(x+3)(3x-1) (m+n)^2-n^2 =(m+n+n)(m+n-n) =(m+2n)m 169(a-b)^2-196(a+b)^2 =(13a-13b+14a+14b)(13a-13b-14a-14b) =-(27a+b)(a+27b) (2x+y)^2-(x+2y)^2 =(2x+y+x+2y)(2x+y-x-2y) =(3x+3y)(x-y) =3(x+y)(x-y) (a+b+c)^2-(a+b-c)^2 =(a+b+c+a+b-c)(a+b+c-a-b+c) =(2a+2b)2c =4c(a+b)