已知x=3y，试求(x-y)^2+2xy-y^2/x^2+7xy+y^2
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[(x-y)^2+2xy-y^2]/[x^2+7xy+y^2]
=[x²-2xy+y²+2xy-y²]/[x^2+7xy+y^2]
=x²/[x^2+7xy+y^2]
x=3y
x²/[x^2+7xy+y^2]
=9y²/[9y²+21y²+y²]
=9y²/31y²
=9/31
=[x²-2xy+y²+2xy-y²]/[x^2+7xy+y^2]
=x²/[x^2+7xy+y^2]
x=3y
x²/[x^2+7xy+y^2]
=9y²/[9y²+21y²+y²]
=9y²/31y²
=9/31
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