The  functions is .

Differentiate with respect to  on each side.

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Substitute  in the above expression.

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Observe the graph of .

From the graph, .

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Find .

 is the slope of the tangent line to the curve  at .

Consider two points on the tangent line to the curve  at .

The two points are  and .

Slope of the tangent line .

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Substitute   in the above expression.

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