已知x-9y=y-3x,化简得x=(5y)/2.两边平方得x^2=(25y^2)/4所以(x^2-y^2)/(x^2+y^2) (用x^2=(25y^2)/4代入左边的式子)=[25y^2)/4-y^2)]/[(25y^2)/4+y^2)]=[(-21y^2)/4]/[(29y^2)/4]=-21/29