24题先化简，再求值
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原式=[x/(x-y)÷x²/(x-y)²][x/(x+y)-x²/(x+y)(x-y)]
=[x/(x-y)*(x-y)²/x²][x(x-y)/(x+y)(x-y)-x²/(x+y)(x-y)]
=[(x-y)/x][(x²-xy-x²)/(x+y)(x-y)]
=[(x-y)/x][-xy/(x+y)(x-y)]
=-y/(x+y)
=-(-3)/(2/3-3)
=-9/7
=[x/(x-y)*(x-y)²/x²][x(x-y)/(x+y)(x-y)-x²/(x+y)(x-y)]
=[(x-y)/x][(x²-xy-x²)/(x+y)(x-y)]
=[(x-y)/x][-xy/(x+y)(x-y)]
=-y/(x+y)
=-(-3)/(2/3-3)
=-9/7
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