$\"\"$

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The function is $\"\"$ .

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The function has horizontal tangent line which has a zero slope.

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Slope of the tangent line is the derivative of the function.

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Equate derivative of the function to zero.

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$\"\"$ .

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

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$\"\"$

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Quotient rule of derivatives : $\"\"$ .

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$\"\"$

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$\"\"$ .

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$\"\"$

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$\"\"$ .

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Equate derivative function to zero.

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$\"\"$ .

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$\"\"$

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

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

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Substitute $\"\"$ in the function.

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$\"\"$ .

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Point of tangency is $\"\"$ .

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Substitute $\"\"$ in the function.

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$\"\"$ .

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Point of tangency is $\"\"$ .

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Hence the points at which the function has horizontal tangent lines are $\"\"$ and $\"\"$ .

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$\"\"$

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