m^2-n^2 = (m+n)(m-n)
x^2+2mx+m^2-n^2=0
(x+m-n)(x+m+n) = 0
x1=n-m x2=-n-m
--
(a^2/4)-b^2 = (a/2+b)(a/2-b)
x^2-ax+(a^2/4)-b^2=0
(x-a/2+b)(x-a/2-b)=0
x1=a/2+b x2=a/2-b
--
x^2-bx-2b^2=0
(x - 2b)(x + b) = 0
x1 = 2b x2 = -b
秘诀就是靠RP...