(a+b)^2=a^2+2ab+b^2
(a-b)^2=a^2-2ab+b^2
∵(a+b)^2
=(a+b)(a+b)
=a^2+ab+ab+b^2
=a^2+2ab+b^2
∵(a-b)^2
=(a-b)(a-b)
=a^2-ab-ab+b^2
=a^2-2ab+b^2
记住头平方,尾平方,头尾的两倍放中央,再根据中间符号来写+ -