Question 16: Given the function \(y = {x^7} – {x^5}\). Which of the following assertion is true

\(\begin{array}{l}

y’ = 7{x^6} – 5{x^4} = {x^4}\left( {7{x^2} – 5} \right) = 0\\

\Leftrightarrow \left[\begin{array}{l}[\begin{array}{l}

x = 0\\

x = \pm \sqrt {\frac{5}{7}}

\end{array} \right.

\end{array}\)

y′ only changes sign when x passes through \( \pm \sqrt {\frac{5}{7}} \) so the function has two extremes

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