$\"\"$

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The function is $\"\"$

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$\"\"$

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Apply derivative on each side with respect to $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$ \ \

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$\"\"$ .

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$\"\"$

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$\"\"$ .

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The function has a horizontal tangent then $\"\"$ .

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Substitute $\"\"$ .

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$\"\"$

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$\"\"$

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Substitute $\"\"$ in the equation $\"\"$ .

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$\"\"$

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$\"\"$

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Take out common term $\"\"$ .

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$\"\"$

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$\"\"$ \ \

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$\"\"$

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$\"\"$ .

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Therefore the function has horizontal tangent at the points $\"\"$ and $\"\"$ .

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$\"\"$

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The function has horizontal tangent at the points $\"\"$ and $\"\"$ .