(2/3a^4b^7-1/9a^2b^6)除以(-1/3ab^3)^2=
(x-y)^2-(x+y)(x-y)=
已知x^2+y^2=25,x+y=7且x>y,则x-y的值等于
先化简，再求值：（a+b）（a-b）+b(b+2)=________=________(其中a=根号2，b=-1）