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JEE Advanced 2025 · Integrals

For all x>0x > 0, let y1(x),y2(x),y3(x)y_{1}(x), y_{2}(x), y_{3}(x) be the functions satisfying dy1dx−(sin⁡x)2 y1=0,y1(1)=5,\frac{dy_{1}}{dx} - (\sin x)^{2}\,y_{1} = 0,\quad y_{1}(1) = 5, dy2dx−(cos⁡x)2 y2=0,y2(1)=13,\frac{dy_{2}}{dx} - (\cos x)^{2}\,y_{2} = 0,\quad y_{2}(1) = \frac{1}{3}, dy3dx−(2−x3x3)y3=0,y3(1)=35e,\frac{dy_{3}}{dx} - \left(\frac{2 - x^{3}}{x^{3}}\right) y_{3} = 0,\quad y_{3}(1) = \frac{3}{5e}, respectively. Then lim⁡x→0+y1(x)y2(x)y3(x)+2xe3xsin⁡x\lim_{x \to 0^{+}} \frac{y_{1}(x)y_{2}(x)y_{3}(x) + 2x}{e^{3x} \sin x} is equal to ________.

Answer: 2.002.00

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Written solution

For all x > 0, let y₁(x), y₂(x), y₃(x) be the functions satisfying dy₁/dx - (sin x)² y₁ = 0, y₁(1) = 5, dy₂/dx - (cos x)² y₂ = 0, y₂(1) = 1/3, dy₃/dx - ((2 - x³)/x³) y₃ = 0, y₃(1) = 3/5e, respectively. Then lim_(x → 0⁺) (y₁(x)y₂(x)y₃(x) + 2x)/(e³ˣ sin x) is equal to ________.

dy1dx−(sin⁡x)2y1=0,y1(1)=5\dfrac{dy_1}{dx} - (\sin x)^2 y_1 = 0, \quad y_1(1)=5

dy2dx−(cos⁡x)2y2=0,y2(1)=13\dfrac{dy_2}{dx} - (\cos x)^2 y_2 = 0, \quad y_2(1)=\dfrac13

dy3dx−(2−x3x3)y3=0,y3(1)=35e\dfrac{dy_3}{dx} - \left(\dfrac{2-x^3}{x^3}\right)y_3 = 0, \quad y_3(1)=\dfrac{3}{5e}

To find: L=lim⁡x→0+y1y2y3+2xe3xsin⁡x\text{To find: } L = \lim_{x\to 0^+} \dfrac{y_1 y_2 y_3 + 2x}{e^{3x}\sin x}

dyidx=qi(x) yi ⇒ ln⁡yi=∫qi(x) dx+Ci\dfrac{dy_i}{dx} = q_i(x)\,y_i \ \Rightarrow \ \ln y_i = \int q_i(x)\,dx + C_i

∫sin⁡2x dx=x2−sin⁡2x4,∫cos⁡2x dx=x2+sin⁡2x4\int \sin^2 x\,dx = \dfrac{x}{2} - \dfrac{\sin 2x}{4}, \quad \int \cos^2 x\,dx = \dfrac{x}{2} + \dfrac{\sin 2x}{4}

ln⁡y1=x2−sin⁡2x4+C1,ln⁡y2=x2+sin⁡2x4+C2\ln y_1 = \dfrac{x}{2} - \dfrac{\sin 2x}{4} + C_1, \quad \ln y_2 = \dfrac{x}{2} + \dfrac{\sin 2x}{4} + C_2

ln⁡(y1y2)=x+(C1+C2),y1(1)y2(1)=53\ln(y_1 y_2) = x + (C_1 + C_2), \quad y_1(1)y_2(1) = \dfrac53

⇒ y1y2=53 e x−1\Rightarrow \ y_1 y_2 = \dfrac{5}{3}\,e^{\,x-1}

ln⁡y3=∫(2x3−1)dx=−1x2−x+C3\ln y_3 = \int \left(\dfrac{2}{x^3} - 1\right)dx = -\dfrac{1}{x^2} - x + C_3

y3=35 e 1−x−1/x2y_3 = \dfrac{3}{5}\,e^{\,1 - x - 1/x^2}

y1y2y3=53ex−1⋅35e1−x−1/x2=e−1/x2y_1 y_2 y_3 = \dfrac53 e^{x-1}\cdot\dfrac35 e^{1-x-1/x^2} = e^{-1/x^2}

L=lim⁡x→0+e−1/x2+2xe3xsin⁡x=lim⁡x→0+e−1/x2/x+2e3x(sin⁡x/x)L = \lim_{x\to 0^+}\dfrac{e^{-1/x^2} + 2x}{e^{3x}\sin x} = \lim_{x\to 0^+}\dfrac{e^{-1/x^2}/x + 2}{e^{3x}(\sin x/x)}

=0+21=2= \dfrac{0 + 2}{1} = 2

2\boxed{2}

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