已知x=3y,试求(x+y)^2+2xy-y^2/x^2+7xy+y^2的值

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已知x=3y,试求(x+y)^2+2xy-y^2/x^2+7xy+y^2的值
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已知x=3y,试求(x+y)^2+2xy-y^2/x^2+7xy+y^2的值
已知x=3y,试求(x+y)^2+2xy-y^2/x^2+7xy+y^2的值

已知x=3y,试求(x+y)^2+2xy-y^2/x^2+7xy+y^2的值
原式=（4y)^2+2*3y*y-y^2/9y^2+7*3y*y+y^2
=16y^2+6y^2-1/9+21y^2+y^2
=44y^2-1/9

直接代入都可以求出来，化简一下就算起来简单点 ：（x+y-y）(x+y+y)+2xy/(x+y)^2+5xy=x(x+2y)+2xy/(x+y)^2+5xy=x^2+2xy+2xy/(x+y)^2+5xy=x^2+4xy/x^2+y^2+7xy代入x=3y得：9y^2+12y^2/10y^2+21y^2=21y^2/31y^2=21/31

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