x²-7xy+12y²=(x-4y)(x-3y)=0x=4y或x=3yx=4y时(x²-xy+y²)/2xy=(16-4+1)y²/8y²=13/8x=3y时(x²-xy+y²)/2xy=(9-3+1)y²/6y²=7/62xy分之x平方-xy+y的值为13/8或7/6...