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JEE Advanced 2012 · Application of Derivatives

If f(x)=∫0xet2(t−2)(t−3) dtf(x)=\int_{0}^{x} e^{t^{2}}(t-2)(t-3)\,dt for all x∈(0,∞)x \in (0,\infty), then

Answers: A. ff has a local maximum at x=2x=2 · B. ff is decreasing on (2,3)(2,3) · C. there exists some c∈(0,∞)c \in (0,\infty), such that f′′(c)=0f''(c)=0 · D. ff has a local minimum at x=3x=3

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If f(x)=∫₀ˣ e^(t²)(t-2)(t-3) dt for all x ∈ (0,∞), then f(x)=∫0xet2(t−2)(t−3) dt,x∈(0,∞)f(x) = \int_0^x e^{t^2}(t-2)(t-3)\,dt, \qquad x \in (0,\infty) Which of (1)-(4) are TRUE?\text{Which of (1)-(4) are TRUE?} 1. f′(x) by FTC1.\ f'(x) \text{ by FTC} 2. sign of f′⇒extrema, monotonicity2.\ \text{sign of } f' \Rightarrow \text{extrema, monotonicity} 3. f′′(x)=0 for some c3.\ f''(x) = 0 \text{ for some } c f′(x)=ex2(x−2)(x−3)(FTC)f'(x) = e^{x^2}(x-2)(x-3) \quad (\text{FTC}) ex2>0⇒sign⁡f′=sign⁡[(x−2)(x−3)]e^{x^2} > 0 \Rightarrow \operatorname{sign} f' = \operatorname{sign}\big[(x-2)(x-3)\big] f′>0 on (0,2)∪(3,∞),f′<0 on (2,3)f' > 0 \text{ on } (0,2)\cup(3,\infty), \qquad f' < 0 \text{ on } (2,3) (1): f′ changes +→− at x=2⇒local maximum⇒TRUE(1):\ f' \text{ changes } + \to - \text{ at } x=2 \Rightarrow \text{local maximum} \Rightarrow \text{TRUE} (2): f′<0 on (2,3)⇒f decreasing on (2,3)⇒TRUE(2):\ f' < 0 \text{ on } (2,3) \Rightarrow f \text{ decreasing on } (2,3) \Rightarrow \text{TRUE} (4): f′ changes −→+ at x=3⇒local minimum⇒TRUE(4):\ f' \text{ changes } - \to + \text{ at } x=3 \Rightarrow \text{local minimum} \Rightarrow \text{TRUE} f′′(x)=ex2[2x(x−2)(x−3)+(2x−5)]=ex2[2x3−10x2+14x−5]f''(x) = e^{x^2}\big[2x(x-2)(x-3) + (2x-5)\big] = e^{x^2}\big[2x^3-10x^2+14x-5\big] p(0)=−5<0,p(1)=1>0⇒∃ c∈(0,1):f′′(c)=0⇒TRUEp(0) = -5 < 0, \quad p(1) = 1 > 0 \Rightarrow \exists\, c \in (0,1): f''(c) = 0 \Rightarrow \text{TRUE} All four (1),(2),(3),(4) are TRUE\boxed{\text{All four } (1), (2), (3), (4) \text{ are TRUE}}

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