已知x^2+3x+7=8,求代数式 3x^2+9x-9 的值
问题描述：
已知x^2+3x+7=8,求代数式 3x^2+9x-9 的值
另一道题
已知x/y = 5/3 ,求x/(x+y) = x/(x-y) - y^2/(x^2-y^2)的值
求详细步骤谢谢
答
x^2+3x+7=8
x^2+3x=1
3x^2+9x-9
=3(x^2+3x)-9
=3-9
=-6
x/(x+y) = x/(x-y) - y^2/(x^2-y^2)
这个是等吗不好意思，加号x/(x+y) + x/(x-y) - y^2/(x^2-y^2)通分=x(x-y)/(x²-y²)+x(x+y)/(x²-y²)-y²/(x²-y²)=[x²-xy+x²+xy-y²]/(x²-y²)=(2x²-y²)/(x²-y²)同时除以y²得=[2(x/y)²-1]/[(x/y)²-1]带入 x/y=5/3=[2*(25/9)-1]/(25/9-1)=41/16