arthham

JEE Advanced 2010 · Integrals

Let ff be a real-valued function defined on the interval (0, ∞)(0,\,\infty) by f(x)=ln⁡x+∫0x1+sin⁡t dtf(x)=\ln x+\int_{0}^{x}\sqrt{1+\sin t}\,dt. Then which of the following statement(s) is (are) true?

Answers: B. f′(x)f'(x) exists for all x∈(0,∞)x\in(0,\infty) and f′f' is continuous on (0,∞)(0,\infty), but not differentiable on (0,∞)(0,\infty) · C. there exists α>1\alpha>1 such that ∣f′(x)∣<∣f(x)∣|f'(x)|<|f(x)| for all x∈(α,∞)x\in(\alpha,\infty)

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f(x)=ln⁡x+∫0x1+sin⁡t dt,x∈(0,∞)f(x) = \ln x + \int_{0}^{x} \sqrt{1+\sin t}\,dt, \qquad x \in (0,\infty) If g(x)=∫0xh(t) dt then g′(x)=h(x)\text{If } g(x) = \int_0^x h(t)\,dt \text{ then } g'(x) = h(x) Check existence/continuity of f′,f′′; and bounds on ∣f∣,∣f′∣\text{Check existence/continuity of } f', f''; \text{ and bounds on } |f|, |f'| f′(x)=1x+1+sin⁡xf'(x) = \dfrac{1}{x} + \sqrt{1+\sin x} 1+sin⁡x=(sin⁡x2+cos⁡x2)21+\sin x = \left(\sin\tfrac{x}{2}+\cos\tfrac{x}{2}\right)^{2} 1+sin⁡x=∣sin⁡x2+cos⁡x2∣\sqrt{1+\sin x} = \left|\sin\tfrac{x}{2}+\cos\tfrac{x}{2}\right| corner where 1+sin⁡x=0, i.e. x=3π2\text{corner where } 1+\sin x = 0,\ \text{i.e. } x = \tfrac{3\pi}{2} f′ continuous but not differentiable at 3π2f' \text{ continuous but not differentiable at } \tfrac{3\pi}{2} ⇒(1) FALSE,(2) TRUE\Rightarrow (1) \text{ FALSE}, \qquad (2) \text{ TRUE} ∣f′(x)∣=∣1x+1+sin⁡x∣≤1+2(x≥1)|f'(x)| = \left|\dfrac{1}{x}+\sqrt{1+\sin x}\right| \leq 1 + \sqrt{2} \quad (x \geq 1) f(x)→∞ (x→∞),f′ bounded⇒∣f′(x)∣<∣f(x)∣ for large xf(x) \to \infty \ (x \to \infty), \quad f' \text{ bounded} \Rightarrow |f'(x)| < |f(x)| \text{ for large } x ⇒(3) TRUE\Rightarrow (3) \text{ TRUE} f(x)→∞⇒no β bounds ∣f(x)∣+∣f′(x)∣⇒(4) FALSEf(x) \to \infty \Rightarrow \text{no } \beta \text{ bounds } |f(x)|+|f'(x)| \Rightarrow (4) \text{ FALSE} TRUE statements: (2) and (3)\boxed{\text{TRUE statements: } (2) \text{ and } (3)} f′(x)=1x+1+sin⁡x;TRUE: (2),(3)f'(x) = \tfrac{1}{x} + \sqrt{1+\sin x}; \quad \text{TRUE: } (2), (3)

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