1.1/x+1/y =(x+y)/xy =-1/(x+y)
去分母可得,（x+y)^2+xy=0 即x^2+y^2=-3xy
所以y/x+x/y=(x^2+y^2)/xy=-3
2.由1/x-1/y=5可得y-x=5xy,即x-y=-5xy
3x+5xy-3y/x-3xy-y=3(x-y)+5xy/x-y-3xy=-10xy/-8xy=5/4
3.ab=1,即a=1/b所以1/（a^2-1）=1/[（1/b^2)-1]=b^2/1-b^2
所以1/a^2-1+1/b^2-1=b^2/1-b^2+1/b^2-1=-1