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Maxima & Minima · Question 43 of 80

Question 43

HardOriginal question

What is the local maximum value of f(x)=−2x3+9x2−12x+7f(x) = - 2 x^{3} + 9 x^{2} - 12 x + 7?

Options for question 43

Show the answer and explanation
CorrectNot quiteAnswer D

Correct answer: D

Explanation

  1. f′(x)=−6(x−2)(x−1)=0f'(x) = - 6 \left(x - 2\right) \left(x - 1\right) = 0 at x=1x = 1 and x=2x = 2.

  2. f′′(x)=18−12xf''(x) = 18 - 12 x: f′′(2)=−6f''(2) = -6, so x=2x = 2 is a local maximum.

  3. The value there is f(2)=3f(2) = 3.

Worth noting

22 is where it happens, not the value; and 22 is the local minimum.

Concept
Local maximum and minimum values
Typical time
120 s
Your time
—
The rule this testsThe second derivative test
Practise this chapter →

Ten questions from Maxima & Minima, at your own pace.

Practise this chapter →

Ten questions from Maxima & Minima, at your own pace.