The interval in which the function $f(x) = 2x^3 + 9x^2 + 12x - 1$ is decreasing is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → $(-2, -1)$ ##
$f(x) = 2x^3 + 9x^2 + 12x - 1$
$f'(x) = 6x^2 + 18x + 12 - 0$
For decreasing function $f'(x) < 0$
$6(x^2 + 3x + 2) < 0$
$6(x + 2)(x + 1) < 0$
Therefore, $f(x)$ is decreasing in interval $(-2, -1)$.