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JEE Advanced 2021 · Applications of Integrals

The area of the region {(x,y):0≤x≤94, 0≤y≤1, x≥3y, x+y≥2}\left\{(x,y): 0 \le x \le \frac{9}{4},\ 0 \le y \le 1,\ x \ge 3y,\ x + y \ge 2\right\} is

Answer: A. 1132\frac{11}{32}

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The area of the region {(x,y): 0 ≤ x ≤ 9/4, 0 ≤ y ≤ 1, x ≥ 3y, x + y ≥ 2} is 0≤x≤94,0≤y≤10 \le x \le \frac{9}{4},\quad 0 \le y \le 1 x≥3y⇒y≤x3;x+y≥2⇒y≥2−xx \ge 3y \Rightarrow y \le \frac{x}{3};\qquad x + y \ge 2 \Rightarrow y \ge 2 - x 2−x≤y≤x3,0≤y≤12 - x \le y \le \frac{x}{3},\qquad 0 \le y \le 1 Find area of this region\text{Find area of this region} x3=2−x⇒x=32,y=12\frac{x}{3} = 2-x \Rightarrow x=\frac{3}{2},\quad y=\frac{1}{2} A=∫3/22(x3−(2−x))dx+∫29/4(x3−0)dxA = \int_{3/2}^{2}\left(\frac{x}{3} - (2-x)\right)dx + \int_{2}^{9/4}\left(\frac{x}{3} - 0\right)dx =∫3/22(4x3−2)dx+∫29/4x3 dx= \int_{3/2}^{2}\left(\frac{4x}{3}-2\right)dx + \int_{2}^{9/4}\frac{x}{3}\,dx ∫(4x3−2)dx=2x23−2x\int\left(\frac{4x}{3}-2\right)dx = \frac{2x^2}{3}-2x [2x23−2x]3/22=−43−(−32)=16\left[\frac{2x^2}{3}-2x\right]_{3/2}^{2} = -\frac{4}{3}-\left(-\frac{3}{2}\right) = \frac{1}{6} ∫29/4x3 dx=[x26]29/4=16(8116−4)=1796\int_2^{9/4}\frac{x}{3}\,dx = \left[\frac{x^2}{6}\right]_2^{9/4} = \frac{1}{6}\left(\frac{81}{16}-4\right) = \frac{17}{96} A=16+1796=16+1796=3396A = \frac{1}{6} + \frac{17}{96} = \frac{16+17}{96} = \frac{33}{96} =1132⇒option (1)= \frac{11}{32} \Rightarrow \text{option (1)}

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