$\"\"$

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The function is $\"\"$ and the interval is $\"\"$ .

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The slope of the horizontal tangent line is zero.

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Evaluate $\"\"$ .

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Consider $\"\"$ .

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Differentiate the function with respect to $\"\"$ .

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$\"\"$

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Sum and difference rule: $\"\"$ .

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$\"\"$

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Constant multiple rule of derivative : $\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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Derivative of the function is $\"\"$ .

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$\"\"$

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Equate $\"\"$ .

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$\"\"$

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If $\"\"$ , the general solution is $\"\"$ where $\"image\"$ is an integer.

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$\"\"$

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If $\"image\"$ , then

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$\"\"$

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If $\"image\"$ , then

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$\"\"$

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If $\"\"$ , then

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$\"\"$

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$\"\"$ is not in the interval $\"\"$ .

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The values of $\"\"$ in the interval $\"\"$ are $\"\"$ and $\"\"$ .

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$\"\"$

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Now substitute corresponding $\"\"$ values in the function .

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Substitute $\"\"$ in $\"\"$ .

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$\"\"$

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Substitute $\"\"$ in $\"\"$ .

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$\"\"$

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The function has a horizontal tangent lines at $\"\"$ and $\"\"$ .

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$\"\"$

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The function has a horizontal tangent lines at $\"\"$ and $\"\"$ .