1（x+y)（a+2b）= ax+2bx+ay+2by
2 (a+b)(a-b)=a^2-ab+ab-b^2=a^2-b^2
3 (a-b)(a-1/3) =a^2-1/3a-ab+1/3b
别的条件没给,只能做到这.