JEE Challenger
More from Continuity and Differentiability

Limit and Derivative Relations for Product Function Involving Sine

Let ff and gg be real valued functions defined on interval (−1,1)(-1, 1) such that g′′(x)g''(x) is continuous, g(0)≠0g(0) \neq 0, g′(0)=0g'(0) = 0, g′′(0)≠0g''(0) \neq 0, and f(x)=g(x)sin⁡xf(x) = g(x) \sin x.

STATEMENT-1 : lim⁡x→0[g(x)cot⁡x−g(0)cosec x]=f′′(0)\lim_{x \to 0} [g(x) \cot x - g(0) \text{cosec } x] = f''(0).

and

STATEMENT-2 : f′(0)=g(0)f'(0) = g(0).

Options

A

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is a correct explanation for STATEMENT-1

Correct
B

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is NOT a correct explanation for STATEMENT-1

C

STATEMENT-1 is True, STATEMENT-2 is False

D

STATEMENT-1 is False, STATEMENT-2 is True

Step-by-Step Solution

To determine the correctness of the statements, we evaluate each statement systematically.

Evaluation of STATEMENT-2

Given the function:
f(x)=g(x)sin⁡xf(x) = g(x) \sin x

Differentiating f(x)f(x) with respect to xx using the product rule:
f′(x)=g′(x)sin⁡x+g(x)cos⁡xf'(x) = g'(x) \sin x + g(x) \cos x

Evaluating at x=0x = 0:
f′(0)=g′(0)sin⁡(0)+g(0)cos⁡(0)f'(0) = g'(0) \sin(0) + g(0) \cos(0)

Since g′(0)=0g'(0) = 0 and cos⁡(0)=1\cos(0) = 1:
f′(0)=0⋅0+g(0)⋅1=g(0)f'(0) = 0 \cdot 0 + g(0) \cdot 1 = g(0)

Thus, STATEMENT-2 is True.


Evaluation of STATEMENT-1

We need to evaluate the limit:
L=lim⁡x→0[g(x)cot⁡x−g(0)csc⁡x]L = \lim_{x \to 0} \left[ g(x) \cot x - g(0) \csc x \right]

Rewriting in terms of sin⁡x\sin x and cos⁡x\cos x:
L=lim⁡x→0g(x)cos⁡x−g(0)sin⁡xL = \lim_{x \to 0} \frac{g(x) \cos x - g(0)}{\sin x}

From STATEMENT-2, we have g(0)=f′(0)g(0) = f'(0). Furthermore, from f′(x)=g′(x)sin⁡x+g(x)cos⁡xf'(x) = g'(x) \sin x + g(x) \cos x, we get:
g(x)cos⁡x=f′(x)−g′(x)sin⁡xg(x) \cos x = f'(x) - g'(x) \sin x

Substituting g(0)=f′(0)g(0) = f'(0) and g(x)cos⁡x=f′(x)−g′(x)sin⁡xg(x) \cos x = f'(x) - g'(x) \sin x into the limit:
L=lim⁡x→0f′(x)−g′(x)sin⁡x−f′(0)sin⁡xL = \lim_{x \to 0} \frac{f'(x) - g'(x) \sin x - f'(0)}{\sin x}

Split the fraction into two terms:
L=lim⁡x→0[f′(x)−f′(0)sin⁡x−g′(x)sin⁡xsin⁡x]L = \lim_{x \to 0} \left[ \frac{f'(x) - f'(0)}{\sin x} - \frac{g'(x) \sin x}{\sin x} \right]
L=lim⁡x→0[f′(x)−f′(0)x⋅xsin⁡x−g′(x)]L = \lim_{x \to 0} \left[ \frac{f'(x) - f'(0)}{x} \cdot \frac{x}{\sin x} - g'(x) \right]

Using standard limit results:

  1. lim⁡x→0f′(x)−f′(0)x=f′′(0)\lim_{x \to 0} \frac{f'(x) - f'(0)}{x} = f''(0) (by definition of the second derivative)
  2. lim⁡x→0xsin⁡x=1\lim_{x \to 0} \frac{x}{\sin x} = 1
  3. lim⁡x→0g′(x)=g′(0)=0\lim_{x \to 0} g'(x) = g'(0) = 0 (since g′′(x)g''(x) is continuous)

Substituting these values:
L=f′′(0)⋅1−0=f′′(0)L = f''(0) \cdot 1 - 0 = f''(0)

Thus, STATEMENT-1 is True.


Conclusion

STATEMENT-1 and STATEMENT-2 are both True, and STATEMENT-2 (f′(0)=g(0)f'(0) = g(0)) is directly used to simplify the expression and prove STATEMENT-1. Hence, STATEMENT-2 is the correct explanation for STATEMENT-1.

Correct Answer: (A)