证明：│x│≤1,│y│≤1,所以x+1≥0,y+1≥0,x-1≤0,y-1≤0,且│xy│≤1,-1≤xy≤1,1+xy≥0,(x+y)/(1+xy)+1 =(x+y+1+xy)/(1+xy )=(x+1)(y+1)/(1+xy)≥0,(x+y)/(1+xy)≥-1,(x+y)/(1+xy)-1 =(x+y-1-xy)/(1+xy) =-(x-1)...