arthham

JEE Advanced 2008 · Continuity and Differentiability

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 I : lim⁡x→0[g(x)cot⁡x−g(0)cosec⁡x]=f′′(0)\lim_{x \to 0} [g(x)\cot x - g(0)\operatorname{cosec} x] = f''(0) and Statement II : f′(0)=g(0)f'(0) = g(0)

Answer: A. Statement I is True, Statement II is True; Statement II is a correct explanation for Statement I

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Written solution

g′′ continuous,g(0)≠0,g′(0)=0,g′′(0)≠0g'' \text{ continuous},\quad g(0)\neq 0,\quad g'(0)=0,\quad g''(0)\neq 0

f(x)=g(x)sin⁡xf(x) = g(x)\sin x

I: lim⁡x→0[g(x)cot⁡x−g(0)cosec⁡x]=f′′(0)\text{I: } \lim_{x\to 0}\left[g(x)\cot x - g(0)\operatorname{cosec} x\right] = f''(0)

II: f′(0)=g(0)\text{II: } f'(0) = g(0)

1. check II: f′(0)1.\ \text{check II: } f'(0)

2. evaluate the limit in I2.\ \text{evaluate the limit in I}

3. compare with f′′(0)3.\ \text{compare with } f''(0)

f′(x)=g′(x)sin⁡x+g(x)cos⁡xf'(x) = g'(x)\sin x + g(x)\cos x

f′(0)=g′(0)⋅0+g(0)⋅1=g(0)⇒II TRUEf'(0) = g'(0)\cdot 0 + g(0)\cdot 1 = g(0) \Rightarrow \text{II TRUE}

g(x)cot⁡x−g(0)cosec⁡x=g(x)cos⁡x−g(0)sin⁡xg(x)\cot x - g(0)\operatorname{cosec} x = \frac{g(x)\cos x - g(0)}{\sin x}

lim⁡x→0g(x)cos⁡x−g(0)sin⁡x=g′(0)cos⁡0−g(0)sin⁡0cos⁡0=01=0\lim_{x\to 0}\frac{g(x)\cos x - g(0)}{\sin x} = \frac{g'(0)\cos 0 - g(0)\sin 0}{\cos 0} = \frac{0}{1} = 0

f′′(x)=g′′(x)sin⁡x+2g′(x)cos⁡x−g(x)sin⁡xf''(x) = g''(x)\sin x + 2g'(x)\cos x - g(x)\sin x

f′′(0)=0f''(0) = 0

lim⁡=0=f′′(0)⇒I TRUE\lim = 0 = f''(0) \Rightarrow \text{I TRUE}

II (f′(0)=g(0)) does not explain I\text{II } (f'(0)=g(0)) \text{ does not explain I}

Both true; II not a correct explanation⇒(2)\boxed{\text{Both true; II not a correct explanation} \Rightarrow (2)}

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