已知x^2+3x+7=8,,求代数式 3x^2+9x-9 的值
答案:2 悬赏:50 手机版
解决时间 2021-03-14 19:43
- 提问者网友：火车头
- 2021-03-14 14:37
已知x^2+3x+7=8,,求代数式 3x^2+9x-9 的值
最佳答案
- 五星知识达人网友：执傲
- 2021-03-14 15:14
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)
这个是等吗追问不好意思，加号追答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
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)
这个是等吗追问不好意思，加号追答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
全部回答
- 1楼网友：七十二街
- 2021-03-14 15:42
已知x^2+3x+7=8,,
x^2+3x=1
代数式 3x^2+9x-9 的值
=3(x^2+3x)-9
=3*1-9
=-6
已知x/y = 5/3 ,
x/(x+y) + x/(x-y) - y^2/(x^2-y^2)的值
=(x(x-y)+x(x+y))/(x+y)(x-y) - y^2/(x^2-y^2)
=2x^2/(x^2-y^2) - y^2/(x^2-y^2)
=(2x^2-y^2)/(x^2-y^2)
=(2(x/y)^2-1)/((x/y)^2-1)
=(2*(5/3)^2-1)/((5/3)^2-1)
=(50/9-1)/(25/9-1)
=41/16追问谢谢了
x^2+3x=1
代数式 3x^2+9x-9 的值
=3(x^2+3x)-9
=3*1-9
=-6
已知x/y = 5/3 ,
x/(x+y) + x/(x-y) - y^2/(x^2-y^2)的值
=(x(x-y)+x(x+y))/(x+y)(x-y) - y^2/(x^2-y^2)
=2x^2/(x^2-y^2) - y^2/(x^2-y^2)
=(2x^2-y^2)/(x^2-y^2)
=(2(x/y)^2-1)/((x/y)^2-1)
=(2*(5/3)^2-1)/((5/3)^2-1)
=(50/9-1)/(25/9-1)
=41/16追问谢谢了
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