MCQ
The interval on which the function f(x) = 2x3 + 9x2 + 12x - 1 is decreasing is:
- A$[-1,\infty)$
- B$[-2,-1]$
- C$(-\infty ,-2]$
- D$[-1,1]$
✓
Answer
Solution:
We have, f(x) = 2x3 + 9x2 + 12x - 1
$\therefore$ f'(x) = 6x2 + 18x + 12
= 6(x2 + 3x + 2) = 6(x + 2)(x + 1)
So, $\text{f}'(\text{x})\leq0,$ for decreasing.
On drawing number lines as below.
We see that f'(x) is decreasing in [-2, -1].
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