$\"\"$

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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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Derivative of a sine function : $\"\"$ .

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

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General solution is $\"\"$ .

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If $\"image\"$ , then

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$\"\"$ .

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

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If $\"image\"$ , then

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$\"\"$

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$\"\"$ and $\"\"$

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$\"\"$ and $\"\"$

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

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Substitute $\"\"$ in the function $\"\"$

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$\"\"$

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The function has a horizontal tangent line at $\"\"$ .

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$\"\"$

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The function has a horizontal tangent line at $\"\"$ .