2a/(a平方-9)+1/(3-a)-3/(a平方+6a+9)
=2a/(a+3)(a-3)-1/(a-3)-3/(a+3)²
=[2a(a+3)-(a+3)²-3(a-3)]/(a-3)(a+3)²
=（2a²+6a-a²-6a-9-3a+9）/(a-3)(a+3)²
=(a²-3a)/(a-3)(a+3)²
=a/(a+3)²
如追加其它问题,