Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Applications of Derivatives

Question:

Let $f(x) = 4x^3- 18x^2 +27x - 5,x∈R$. Then which of the following statements are TRUE?

(A) $f''(x) = 24x - 36$
(B) $f$ has local maxima at $x =\frac{3}{2}$ but no minima
(C) $f$ has neither maxima nor minima
(D) $f$ has both maxima and minima

Choose the correct answer from the options given below:

Options:

(A) and (B) only

(A) and (C) only

(B) and (C) only

(A) and (D) only

Correct Answer:

(A) and (C) only

Explanation:

The correct answer is Option (2) → (A) and (C) only

$f(x)=4x^{3}-18x^{2}+27x-5$

$f'(x)=12x^{2}-36x+27=12\left(x-\frac{3}{2}\right)^{2}\ge 0$

$f''(x)=24x-36$

Critical point: $f'(x)=0 \Rightarrow x=\frac{3}{2}$ and $f''\!\left(\frac{3}{2}\right)=0$.

Since $f'(x)\ge 0$ for all $x$ and vanishes only at $x=\frac{3}{2}$, $f$ is strictly increasing with a stationary inflection; no local extrema.

True statements: (A) and (C)