1、B=(x-3-3-x)/(x^2-9)=6/(9-x^2)
A+B=[4(9-x^2)+2x*（x^2-4）]/(x^2-4)(x^2-9)
2、(y+1)/(x+1)+(x+1)/(y+1)
=[(y+1)(y+1)+(x+1)(x+1)]/[(x+1)(y+1)]
=(y^2+2y+1+x^2+2x+1)/(xy+x+y+1)
=[(x+y)^2-2xy+2x+2y+2]/(1+2+1)
=(4-2+4+2)/4
=2
3、(a-b)/(b-c)(c-a)+(b-c)/(a-b)(c-a)+(c-a)/(a-b)(b-c)
=[(a-b)^2+(b-c)^2+(c-a)^2]/(a-c)(a-b)(b-c)
要是上式等于0,则必须（a-b)^2+(b-c)^2+(c-a)^2=0
可得a=b=c
但是此时分式无意义
则上市不可能为0