A=(x+y)/(1+x+y)=1-1/(1+x+y)
B=x/(1+x)+y/(1+y)=（x+2xy+y)/[(1+x)(1+y)
=（x+2xy+y)/(1+x+y+xy)=1-(1-xy)/(1+x+y+xy)
因为(1-xy)/(1+x+y+xy)<1/(1+x+y+xy)<1/(1+x+y)
所以-(1-xy)/(1+x+y+xy)>-1/(1+x+y)
故B>A