$\"\"$

\

The function is $\"S$

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$\"fraction$ (Apply derivative with respect to r)

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$\"S$ (The constant multiply rule)

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$\"Syntax$ (Rewrite the expression)

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$\"Syntax$ (The power rule)

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$\"Syntax$ (Simplify)

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$\"S$ (Rewrite the expression)

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$\"S$ (Simplify)

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$\"\"$

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To find the critical number then S\\' = 0. \ \

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$\"fraction$ (S\\' = 0)

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$\"minus$ (Multiply each side by $\"italic$ ) \ \

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$\"\"$

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Therefore the critical numbers are undefine. \ \