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JEE Advanced 2018 · Continuity and Differentiability

For every twice differentiable function f:R→[−2,2]f:\mathbb{R}\to[-2,2] with (f(0))2+(f′(0))2=85(f(0))^{2}+(f'(0))^{2}=85, which of the following statement(s) is (are) TRUE?

Answers: A. There exist r,s∈Rr,s\in\mathbb{R}, where r<sr<s, such that ff is one-one on the open interval (r,s)(r,s) · B. There exists x0∈(−4,0)x_{0}\in(-4,0) such that ∣f′(x0)∣≤1|f'(x_{0})|\le 1 · D. There exists α∈(−4,4)\alpha\in(-4,4) such that f(α)+f′′(α)=0f(\alpha)+f''(\alpha)=0 and f′(α)≠0f'(\alpha)\ne 0

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f:R→[−2,2] twice differentiablef: \mathbb{R} \to [-2,2] \text{ twice differentiable} (f(0))2+(f′(0))2=85(f(0))^2 + (f'(0))^2 = 85 Which of (1) - (4) is/are TRUE?\text{Which of (1) - (4) is/are TRUE?} f:R→[−2,2],(f(0))2+(f′(0))2=85f: \mathbb{R} \to [-2,2],\quad (f(0))^2 + (f'(0))^2 = 85 ∣f(0)∣≤2⇒(f′(0))2=85−(f(0))2≥81|f(0)| \le 2 \Rightarrow (f'(0))^2 = 85 - (f(0))^2 \ge 81 ⇒∣f′(0)∣≥9≠0\Rightarrow |f'(0)| \ge 9 \ne 0 (1) local monotonicity(1)\ \text{local monotonicity} (2) MVT on [−4,0](2)\ \text{MVT on } [-4,0] (3) limit at ∞;(4) h=f2+f′2(3)\ \text{limit at } \infty;\quad (4)\ h = f^2 + f'^2 f′(0)≠0, f′ continuous⇒f′ keeps sign near 0f'(0) \ne 0,\ f' \text{ continuous} \Rightarrow f' \text{ keeps sign near } 0 ⇒f one-one on some (r,s)⇒(1) TRUE\Rightarrow f \text{ one-one on some } (r,s) \Rightarrow (1) \text{ TRUE} MVT on [−4,0]: f(0)−f(−4)=4f′(x0)\text{MVT on } [-4,0]:\ f(0)-f(-4) = 4 f'(x_0) ∣f(0)−f(−4)∣≤4⇒∣f′(x0)∣≤1⇒(2) TRUE|f(0)-f(-4)| \le 4 \Rightarrow |f'(x_0)| \le 1 \Rightarrow (2) \text{ TRUE} lim⁡x→∞f(x)=1 is not forced⇒(3) FALSE\lim_{x\to\infty} f(x) = 1 \text{ is not forced} \Rightarrow (3) \text{ FALSE} h(x)=f(x)2+f′(x)2,h(0)=85h(x) = f(x)^2 + f'(x)^2,\quad h(0) = 85 h′(x)=2f′(x) (f(x)+f′′(x))h'(x) = 2f'(x)\,(f(x) + f''(x)) c1∈(−4,0), c2∈(0,4): ∣f′(ci)∣≤1⇒h(ci)≤5c_1\in(-4,0),\ c_2\in(0,4):\ |f'(c_i)| \le 1 \Rightarrow h(c_i) \le 5 max⁡[c1,c2]h≥h(0)=85>5⇒interior max at α\max_{[c_1,c_2]} h \ge h(0) = 85 > 5 \Rightarrow \text{interior max at } \alpha h′(α)=0, f′(α)2≥85−4=81≠0⇒f(α)+f′′(α)=0h'(\alpha)=0,\ f'(\alpha)^2 \ge 85-4 = 81 \ne 0 \Rightarrow f(\alpha)+f''(\alpha)=0 (1), (2), (4) are TRUE\boxed{(1),\ (2),\ (4) \text{ are TRUE}}

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