EXAMPLE 6 Discuss the curve y = 3x* - 48x³ with respect to concavity, points of inflection, and local maxima and minima. Use this information to sketch the curve. SOLUTION If f(x) = 3x* – 48x², then f'(x) = 12x3 - 144x² = 12x²(x – 12) f"(x) = 36x2 – 288x = 36x(x – 8). To find the critical numbers we set f'(x) = 0 and obtain x = 0 and x = | To use the Second Derivative Test we evaluate " at these critical numbers: f"(0 : f"(12) Since f'(12) = and f"(12) > 0, (12) = O is a local minimum. Since f"(0) =O, the Second Derivative Test gives no information about the critical number 0. But since f'(x) < 0 for x < 0 and also for 0

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EXAMPLE 6
Discuss the curve y = 3x* – 48x with respect to concavity, points of inflection, and local maxima and minima. Use this information to
sketch the curve.
SOLUTION
If f(x) = 3x4 - 48x³, then
144x2 = 12x2(x – 12)
288х 3D 36x(х — 8).
f'(x)
12x3
f"(x) = 36x2
8
12
To find the critical numbers we set f'(x) = 0 and obtain x = 0 and x =
To use the Second Derivative Test we evaluate f" at these critical
numbers:
f"(0)
f"(12)
Video Example
Since f'(12)
and f"(12) > 0, f(12) =
is a local minimum. Since f"(0)
the Second Derivative Test gives no information
%3D
about the critical number 0. But since f'(x) < 0 for x < 0 and also for 0 < x < 12, the First Derivative Test tells us that f does not have a local
maximum or minimum at 0. [In fact, the expression for f'(x) shows that f decreases to the left of 12 and increase to the
of 12.]
Since f"(x) = 0 when x = 0 or x =
, we divide the real number line into intervals with these numbers as endpoints and complete the following
chart.
Interval
f"(x)
36x(х — 8) Concavity
(-0, 0)
upward
downward
+
upward
The point (0, 0) is an inflection point since the curve changes from concave upward to concave downward there. Also,
-12288
is an
inflection point since the curve changes from concave downward to concave upward there.
Using the local minimum, the intervals of concavity, and the inflection points, we sketch the curve in the figure.
Transcribed Image Text:y EXAMPLE 6 Discuss the curve y = 3x* – 48x with respect to concavity, points of inflection, and local maxima and minima. Use this information to sketch the curve. SOLUTION If f(x) = 3x4 - 48x³, then 144x2 = 12x2(x – 12) 288х 3D 36x(х — 8). f'(x) 12x3 f"(x) = 36x2 8 12 To find the critical numbers we set f'(x) = 0 and obtain x = 0 and x = To use the Second Derivative Test we evaluate f" at these critical numbers: f"(0) f"(12) Video Example Since f'(12) and f"(12) > 0, f(12) = is a local minimum. Since f"(0) the Second Derivative Test gives no information %3D about the critical number 0. But since f'(x) < 0 for x < 0 and also for 0 < x < 12, the First Derivative Test tells us that f does not have a local maximum or minimum at 0. [In fact, the expression for f'(x) shows that f decreases to the left of 12 and increase to the of 12.] Since f"(x) = 0 when x = 0 or x = , we divide the real number line into intervals with these numbers as endpoints and complete the following chart. Interval f"(x) 36x(х — 8) Concavity (-0, 0) upward downward + upward The point (0, 0) is an inflection point since the curve changes from concave upward to concave downward there. Also, -12288 is an inflection point since the curve changes from concave downward to concave upward there. Using the local minimum, the intervals of concavity, and the inflection points, we sketch the curve in the figure.
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