(a+b)^2=a^2+2ab+b^2

(a-b)^2=a^2-2ab+b^2

因为(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