3x-4y＝z,
2x+y＝8z,
解得：x=3z,y=2z
xy+yz分之x二次方+y二次方-z二次方
=(x^2+y^2-z^2)/(xy+yz)
=(9z^2+4z^2-z^2)/(6z^2+2z^2)
=12/8
=3/2