EXAMPLE 1 Find where the function f(x) = 3x – 16x3 – 270x² + 7 is increasing and where it is decreasing.

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EXAMPLE 1
Find where the function f(x) = 3x4 – 16x3 – 270x2 + 7 is increasing and where it is
-5
10
decreasing.
- 2000
12:(x -O)(*+D)
-4000
SOLUTION
f'(x) = 12x3 – 48x² – 540x =
- 6000
-8000
To use the I/D Test, we have to know where f'(x) > 0 and where f'(x) < 0. This depends on the signs of
the three factors of f'(x), namely, 12x, x –
-10 000
and x +
We divide the real line into intervals
-12 000
whose endpoints are the critical numbers
(smallest), 0 and
(largest) and arrange our work in a
-14 000
chart. A plus sign indicates that the given expression is positive, and a negative sign indicates that it is
negative. The last column of the chart gives the conclusion based on the I/D Test. For instance, f '(x) < 0 for
0 < x < 9, so f is ---Select--- v on (0, 9). (It would also be true to say that f is decreasing on the closed
interval [0, 9].)
Video Example
Interval
12x
х— 9
X + 5
f'(x)
f
х< -5
decreasing on (-∞, -5)
-5 < x < 0
---Select--- ♥
on (-5, 0)
+
+
0 < x < 9
+
decreasing on (0, 9)
x > 9
+
+
+
+
V ---Select---
(9, о»)
decreasing
increasing
The graph of f shown in the figure confirms L.
the chart.
Transcribed Image Text:EXAMPLE 1 Find where the function f(x) = 3x4 – 16x3 – 270x2 + 7 is increasing and where it is -5 10 decreasing. - 2000 12:(x -O)(*+D) -4000 SOLUTION f'(x) = 12x3 – 48x² – 540x = - 6000 -8000 To use the I/D Test, we have to know where f'(x) > 0 and where f'(x) < 0. This depends on the signs of the three factors of f'(x), namely, 12x, x – -10 000 and x + We divide the real line into intervals -12 000 whose endpoints are the critical numbers (smallest), 0 and (largest) and arrange our work in a -14 000 chart. A plus sign indicates that the given expression is positive, and a negative sign indicates that it is negative. The last column of the chart gives the conclusion based on the I/D Test. For instance, f '(x) < 0 for 0 < x < 9, so f is ---Select--- v on (0, 9). (It would also be true to say that f is decreasing on the closed interval [0, 9].) Video Example Interval 12x х— 9 X + 5 f'(x) f х< -5 decreasing on (-∞, -5) -5 < x < 0 ---Select--- ♥ on (-5, 0) + + 0 < x < 9 + decreasing on (0, 9) x > 9 + + + + V ---Select--- (9, о») decreasing increasing The graph of f shown in the figure confirms L. the chart.
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