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JEE Advanced 2016 · Application of Derivatives

Let f:R→(0,∞)f: \mathbb{R} \to (0, \infty) and g:R→Rg: \mathbb{R} \to \mathbb{R} be twice differentiable functions such that f′′f'' and g′′g'' are continuous functions on R\mathbb{R}. Suppose f′(2)=g(2)=0f'(2) = g(2) = 0, f′′(2)≠0f''(2) \neq 0 and g′(2)≠0g'(2) \neq 0. If lim⁡x→2f(x)g(x)f′(x)g′(x)=1\lim_{x \to 2} \frac{f(x)g(x)}{f'(x)g'(x)} = 1, then

Answers: A. ff has a local minimum at x=2x=2 · D. f(x)−f′′(x)=0f(x) - f''(x) = 0 for at least one x∈Rx \in \mathbb{R}

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Let f: R → (0, ∞) and g: R → R be twice differentiable functions such that f'' and g'' are continuous functions on R. Suppose f'(2) = g(2) = 0, f''(2) ≠ 0 and g'(2) ≠ 0. If lim_(x → 2) f(x)g(x)/f'(x)g'(x) = 1, then f′(2)=0,g(2)=0,f′′(2)≠0,g′(2)≠0f'(2) = 0, \quad g(2) = 0, \quad f''(2) \neq 0, \quad g'(2) \neq 0 f>0;f′′,g′′ continuous on Rf > 0; \quad f'', g'' \text{ continuous on } \mathbb{R} lim⁡x→2f(x)g(x)f′(x)g′(x)=1\lim_{x\to 2}\frac{f(x)g(x)}{f'(x)g'(x)} = 1 Determine which statements hold\text{Determine which statements hold} f′(2)=0⇒f′(x)=f′′(2)(x−2)+o(x−2)f'(2) = 0 \Rightarrow f'(x) = f''(2)(x-2) + o(x-2) g(2)=0⇒g(x)=g′(2)(x−2)+o(x−2)g(2) = 0 \Rightarrow g(x) = g'(2)(x-2) + o(x-2) f(x)=f(2)+12f′′(2)(x−2)2+o((x−2)2)f(x) = f(2) + \tfrac{1}{2}f''(2)(x-2)^2 + o((x-2)^2) f(x)g(x)f′(x)g′(x)≈f(2)⋅g′(2)(x−2)f′′(2)(x−2)⋅g′(2)\frac{f(x)g(x)}{f'(x)g'(x)} \approx \frac{f(2)\cdot g'(2)(x-2)}{f''(2)(x-2)\cdot g'(2)} =f(2)f′′(2)= \frac{f(2)}{f''(2)} lim⁡x→2f(x)g(x)f′(x)g′(x)=f(2)f′′(2)=1\lim_{x\to2}\frac{f(x)g(x)}{f'(x)g'(x)} = \frac{f(2)}{f''(2)} = 1 ⇒f′′(2)=f(2)\Rightarrow f''(2) = f(2) f(2)>0⇒f′′(2)=f(2)>0f(2) > 0 \Rightarrow f''(2) = f(2) > 0 f′(2)=0, f′′(2)>0⇒local minimum at x=2f'(2) = 0, \ f''(2) > 0 \Rightarrow \text{local minimum at } x = 2 (3):f′′(2)>f(2) is FALSE since f′′(2)=f(2)(3): f''(2) > f(2) \text{ is FALSE since } f''(2) = f(2) (4):f(2)−f′′(2)=0⇒f(x)−f′′(x)=0 at x=2 ✓(4): f(2) - f''(2) = 0 \Rightarrow f(x) - f''(x) = 0 \text{ at } x = 2 \ \checkmark Correct statements: (1) and (4)\text{Correct statements: } (1) \text{ and } (4)

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