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JEE Advanced 2007 · Integrals

Let F(x)F(x) be an indefinite integral of sin⁡2x\sin^2 x. \ STATEMENT 1 : The function F(x)F(x) satisfies F(x+π)=F(x)F(x+\pi) = F(x) for all real x. because \ STATEMENT 2 : sin⁡2(x+π)=sin⁡2x\sin^2(x+\pi) = \sin^2 x for all real x.

Answer: D. Statement 1 is False, Statement 2 is True.

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Let F(x) be an indefinite integral of sin² x. STATEMENT 1 : The function F(x) satisfies F(x+π) = F(x) for all real x. because STATEMENT 2 : sin²(x+π) = sin² x for all real x. F(x)=∫sin⁡2x dxF(x) = \displaystyle\int \sin^{2}x\,dx Statement 1: F(x+π)=F(x) ∀x∈R\text{Statement 1: } F(x+\pi) = F(x) \ \forall x \in \mathbb{R} Statement 2: sin⁡2(x+π)=sin⁡2x ∀x∈R\text{Statement 2: } \sin^{2}(x+\pi) = \sin^{2}x \ \forall x \in \mathbb{R} To find: truth of both statements, and whether (2) explains (1)\text{To find: truth of both statements, and whether (2) explains (1)} 1. find F(x)=∫sin⁡2x dx1.\ \text{find } F(x) = \displaystyle\int \sin^{2}x\,dx 2. check Statement 2: sin⁡2(x+π)2.\ \text{check Statement 2: } \sin^{2}(x+\pi) 3. check Statement 1: compare F(x+π) with F(x)3.\ \text{check Statement 1: compare } F(x+\pi) \text{ with } F(x) F′(x)=sin⁡2x,F(x)=∫sin⁡2x dxF'(x) = \sin^{2}x, \qquad F(x) = \displaystyle\int \sin^{2}x\,dx sin⁡2x=1−cos⁡2x2\sin^{2}x = \dfrac{1 - \cos 2x}{2} F(x)=12∫(1−cos⁡2x) dx=x2−sin⁡2x4+CF(x) = \dfrac{1}{2}\displaystyle\int(1-\cos 2x)\,dx = \dfrac{x}{2} - \dfrac{\sin 2x}{4} + C sin⁡(x+π)=−sin⁡x⇒sin⁡2(x+π)=sin⁡2x⇒Statement 2 is TRUE\sin(x+\pi) = -\sin x \Rightarrow \sin^{2}(x+\pi) = \sin^{2}x \Rightarrow \text{Statement 2 is TRUE} F(x+π)=x+π2−sin⁡(2x+2π)4+CF(x+\pi) = \dfrac{x+\pi}{2} - \dfrac{\sin(2x+2\pi)}{4} + C =x2+π2−sin⁡2x4+C=F(x)+π2= \dfrac{x}{2} + \dfrac{\pi}{2} - \dfrac{\sin 2x}{4} + C = F(x) + \dfrac{\pi}{2} F(x+π)−F(x)=π2≠0⇒Statement 1 is FALSEF(x+\pi) - F(x) = \dfrac{\pi}{2} \neq 0 \Rightarrow \text{Statement 1 is FALSE} Mean of sin⁡2x over a period=12≠0⇒a linear term x2 survives\text{Mean of } \sin^{2}x \text{ over a period} = \tfrac12 \neq 0 \Rightarrow \text{a linear term } \tfrac{x}{2} \text{ survives} Statement 1 False, Statement 2 True⇒(4)\text{Statement 1 False, Statement 2 True} \Rightarrow (4) Answer: (4)\boxed{\text{Answer: (4)}}

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