Section 4.5 : The Shape of a Graph, Part I
5. For f(x)=2x3−9x2−60xf(x)=2x3−9x2−60x answer each of the following questions.
- Identify the critical points of the function.
- Determine the intervals on which the function increases and decreases.
- Classify the critical points as relative maximums, relative minimums or neither.
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a Identify the critical points of the function. Show SolutionWe need the 1st derivative to get the critical points so here it is.
f′(x)=6x2−18x−60=6(x2−3x−10)=6(x−5)(x+2)Now, recall that critical points are where the derivative doesn’t exist or is zero. Clearly this derivative exists everywhere (it’s a polynomial….) and because we factored the derivative we can easily identify where the derivative is zero. The critical points of the function are,
x=−2,x=5b Determine the intervals on which the function increases and decreases. Show Solution
To determine the increase/decrease information for the function all we need is a quick number line for the derivative. Here is the number line.
From this we get the following increasing/decreasing information for the function.
Increasing : (−∞,−2)&(5,∞)Decreasing : (−2,5)c Classify the critical points as relative maximums, relative minimums or neither. Show Solution
With the increasing/decreasing information from the previous step we can easily classify the critical points using the 1st derivative test. Here is classification of the functions critical points.
x=−2: Relative Maximumx=5: Relative Minimum