已知l是函数f(x)=4x/(x^2+1)的图像上任意一点处的切线,求切线斜率范围

函数的点满足函数f(x)=4x/(x^2+1)求导可得点的斜率斜率k=f ′ (x)=（4x/(x^2+1)）′= [（4x）′(x^2+1) -4x(x^2+1)′ ] /(x^2+1) ^2 【（u/v)'=(u'v-uv')/v²】=（4-4x^2）/(x^2+1) ^2即k=（4-4x^2）/(x^2+1) ^2...