Match List-I with List-II
The function $f(x) = (x − 1)(x + 1)^2$ has
|
List-I |
List-II |
|
(A) A local maxima at x = ____ |
(I) $\frac{1}{3}$ |
|
(B) A local minima at x = ____ |
(II) 0 |
|
(C) The local minimum value of f(x) = ____ |
(III) -1 |
|
(D) The local maximum value of f(x) = ____ |
(IV) $-\frac{32}{27}$ |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) - (III), (B) - (I), (C) - (IV), (D) - (II)
Given: $f(x) = (x - 1)(x + 1)^2$
Compute first derivative:
$f'(x) = \frac{d}{dx}[(x - 1)(x + 1)^2]$
$= (x - 1) \cdot \frac{d}{dx}[(x + 1)^2] + (x + 1)^2 \cdot \frac{d}{dx}[x - 1]$
$= (x - 1)(2)(x + 1) + (x + 1)^2(1)$
$= 2(x - 1)(x + 1) + (x + 1)^2$
$= (x + 1)[2(x - 1) + (x + 1)]$
$= (x + 1)[2x - 2 + x + 1] = (x + 1)(3x - 1)$
Critical points from $f'(x) = 0$:
$(x + 1)(3x - 1) = 0 \Rightarrow x = -1,\ \frac{1}{3}$
Compute second derivative:
$f''(x) = \frac{d}{dx}[(x + 1)(3x - 1)] = (x + 1)(3) + (3x - 1)(1)$
$= 3(x + 1) + (3x - 1) = 3x + 3 + 3x - 1 = 6x + 2$
At $x = -1$: $f''(-1) = 6(-1) + 2 = -4$ ⟹ local maxima
At $x = \frac{1}{3}$: $f''\left(\frac{1}{3}\right) = 6 \cdot \frac{1}{3} + 2 = 4$ ⟹ local minima
Compute $f(-1) = (-1 - 1)(-1 + 1)^2 = (-2)(0)^2 = 0$
Compute $f\left(\frac{1}{3}\right) = \left(\frac{1}{3} - 1\right)\left(\frac{1}{3} + 1\right)^2 = \left(-\frac{2}{3}\right)\left(\frac{4}{3}\right)^2 = -\frac{2}{3} \cdot \frac{16}{9} = -\frac{32}{27}$
|
List-I |
List-II |
|
(A) A local maxima at x = ____ |
(III) -1 |
|
(B) A local minima at x = ____ |
(I) $\frac{1}{3}$ |
|
(C) The local minimum value of f(x) = ____ |
(IV) $-\frac{32}{27}$ |
|
(D) The local maximum value of f(x) = ____ |
(II) 0 |