(x+y)的平方-(3x-2y)的平方=[(x+y)-(3x-2y)][(x+y)+(3x-2y)]=(-2x+3y)(4x-y)=(4x-y)(3y-2x)
(x+y)^2-(3x-2y)^2=(x+y+3x-2y)(x+y-3x+2y)=(4x-y)(3y-2x)
