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JEE Advanced 2022 · Applications of Integrals

Consider the functions f,g:R→Rf, g : \mathbb{R} \to \mathbb{R} defined by f(x)=x2+512f(x) = x^{2} + \frac{5}{12} and g(x)={2(1−4∣x∣3),∣x∣≤34,0,∣x∣>34.g(x) = \begin{cases} 2\left(1 - \frac{4|x|}{3}\right), & |x| \leq \frac{3}{4}, \\ 0, & |x| > \frac{3}{4}. \end{cases} If α\alpha is the area of the region {(x,y)∈R×R:∣x∣≤34,0≤y≤min⁡{f(x),g(x)}},\left\{(x, y) \in \mathbb{R} \times \mathbb{R} : |x| \leq \frac{3}{4}, 0 \leq y \leq \min\{f(x), g(x)\}\right\}, then the value of 9α9\alpha is ______________.

Answer: 66

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Written solution

f(x)=x2+512f(x) = x^2 + \dfrac{5}{12} g(x)={2(1−4∣x∣3),∣x∣≤34,0,∣x∣>34.g(x) = \begin{cases} 2\left(1 - \dfrac{4|x|}{3}\right), & |x| \le \dfrac{3}{4}, \\ 0, & |x| > \dfrac{3}{4}. \end{cases} α=area where ∣x∣≤34, 0≤y≤min⁡{f(x),g(x)};find 9α\alpha = \text{area where } |x|\le \dfrac{3}{4},\ 0\le y\le \min\{f(x),g(x)\}; \quad \text{find } 9\alpha f,g even⇒α=2∫03/4min⁡{f,g} dxf, g \text{ even} \Rightarrow \alpha = 2\int_0^{3/4} \min\{f,g\}\,dx f=g: 12x2+32x−19=0⇒x=12f=g:\ 12x^2+32x-19=0 \Rightarrow x=\dfrac{1}{2} x=0: f=512<2=g⇒min⁡=f on [0,12]x=0:\ f=\dfrac{5}{12}<2=g \Rightarrow \min = f \text{ on } \left[0,\tfrac12\right] x=34: g=0<f⇒min⁡=g on [12,34]x=\dfrac{3}{4}:\ g=0<f \Rightarrow \min = g \text{ on } \left[\tfrac12,\tfrac34\right] ∫01/2(x2+512)dx=[x33+5x12]01/2=124+524=14\int_0^{1/2}\left(x^2+\dfrac{5}{12}\right)dx = \left[\dfrac{x^3}{3}+\dfrac{5x}{12}\right]_0^{1/2} = \dfrac{1}{24}+\dfrac{5}{24} = \dfrac{1}{4} ∫1/23/4(2−8x3)dx=[2x−4x23]1/23/4=34−23=112\int_{1/2}^{3/4}\left(2-\dfrac{8x}{3}\right)dx = \left[2x-\dfrac{4x^2}{3}\right]_{1/2}^{3/4} = \dfrac{3}{4}-\dfrac{2}{3} = \dfrac{1}{12} α=2(14+112)=2⋅13=23\alpha = 2\left(\dfrac{1}{4}+\dfrac{1}{12}\right) = 2\cdot\dfrac{1}{3} = \dfrac{2}{3} 9α=9⋅23=69\alpha = 9\cdot\dfrac{2}{3} = 6 9α=6\boxed{9\alpha = 6}

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