(a+b)(a-b)= a^2-b^2
(a+b)^2=a^2+2ab+b^2
a^m/a^n=a^(m-n)
(2m-n) (-n-2m)=n^2-4m^2
2a^2-2ab=2a(a-b)
如果是这样:(2a)^2-2ab=2a(2a-b)