Mathematics

Problem

For the function

r(x)=\dfrac{x^{3}-4x}{x^{2}+1}

, at which

x

-values does the graph of

r

have a horizontal tangent line? a)

x=\pm\sqrt{\dfrac{7+\sqrt{65}}{2}}

x=\pm\sqrt{\dfrac{-7+\sqrt{65}}{2}}

x=\pm\dfrac{2}{\sqrt{3}}

x=\dfrac{-7+\sqrt{65}}{2}