Connect the points and to draw the secant lines.

According to mean value theorem the function should have a number  in  such that .

From the concept of maxima and minima, the derivative at the maxima or minima are always zero.

Now observe the graph :

The graph has one minima and one maxima at  and respectively.

Draw the tangent lines at and .

Graph of the function using secant and tangent lines.

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