arthham

JEE Advanced 2009 · Applications of Integrals

Let f be a non-negative function defined on the interval [0,1]. If integral0xsqrt(1−(f′(t))2)_{0}^{x} sqrt(1-(f'(t))^{2}) dt == integral0xf(t)_{0}^{x} f(t) dt,0≤x≤1,, 0 \le x \le 1, and f(0)=0,f(0)=0, then (1)f(12)<12(1) f(\frac{1}{2})<\frac{1}{2} and f(13)>13(2)f(12)>12f(\frac{1}{3})>\frac{1}{3} (2) f(\frac{1}{2})>\frac{1}{2} and f(13)>13(3)f(12)<12f(\frac{1}{3})>\frac{1}{3} (3) f(\frac{1}{2})<\frac{1}{2} and f(13)<13(4)f(12)>12f(\frac{1}{3})<\frac{1}{3} (4) f(\frac{1}{2})>\frac{1}{2} and f(13)<13f(\frac{1}{3})<\frac{1}{3}

Answer: C. f(12)<12f\left(\frac{1}{2}\right)<\frac{1}{2} and f(13)<13f\left(\frac{1}{3}\right)<\frac{1}{3}

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∫0x1−(f′(t))2 dt=∫0xf(t) dt,0≤x≤1\int_0^x \sqrt{1-(f'(t))^2}\,dt = \int_0^x f(t)\,dt, \quad 0 \leq x \leq 1 f≥0 on [0,1],f(0)=0f \geq 0 \text{ on } [0,1], \qquad f(0) = 0 Which of (1)-(4) is TRUE?\text{Which of (1)-(4) is TRUE?} 1. differentiate both sides (FTC)1.\ \text{differentiate both sides (FTC)} 2. solve for f2.\ \text{solve for } f 3. compare f(1/2),f(1/3) with 1/2,1/33.\ \text{compare } f(1/2), f(1/3) \text{ with } 1/2, 1/3 ddx:1−(f′(x))2=f(x)\dfrac{d}{dx}: \quad \sqrt{1-(f'(x))^2} = f(x) 1−(f′)2=f2⇒f2+(f′)2=1,f(0)=0, f≥0⇒f(t)=sin⁡t1 - (f')^2 = f^2 \Rightarrow f^2 + (f')^2 = 1, \quad f(0) = 0, \ f \geq 0 \Rightarrow f(t) = \sin t f ⁣(12)=sin⁡12<12,f ⁣(13)=sin⁡13<13(sin⁡x<x)f\!\left(\tfrac{1}{2}\right) = \sin\tfrac{1}{2} < \tfrac{1}{2}, \qquad f\!\left(\tfrac{1}{3}\right) = \sin\tfrac{1}{3} < \tfrac{1}{3} \quad (\sin x < x) ⇒option (3)\Rightarrow \text{option } (3)

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