Find The Interval In Which f (x) = x3 – 3x2 – 9x + 20 Is Strictly Increasing Or Strictly Decreasing.

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Question

Find the interval in which f (x) = x3 – 3x2 – 9x + 20 is strictly increasing or strictly decreasing.

Solution

Correct option is

Strictly increasing and strictly decreasing x Ïµ (–1, 3) 

 

Given                    f (x) = x3 – 3x2 – 9x + 20

                                         

  

  

Using number line method, as shown in figure

Thus, f (x) is strictly increasing for  and strictly decreasing for x Ïµ (–1, 3).

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