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JEE Advanced 2024 · Applications of Integrals

Let S={(x,y)∈R×R:x≥0, y≥0, y2≤4x, y2≤12−2xS=\{(x,y)\in\mathbb{R}\times\mathbb{R}:x\geq 0,\,y\geq 0,\,y^{2}\leq 4x,\,y^{2}\leq 12-2x and 3y+8x≤58}3y+\sqrt{8x}\leq 5\sqrt{8}\}. If the area of the region SS is a2a\sqrt{2}, then α\alpha is equal to

Answer: B. 173\frac{17}{3}

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Let S={(x,y)∈R×R:x≥ 0, y≥ 0, y²≤ 4x, y²≤ 12-2x and 3y+√8x≤ 5√8}. If the area of the region S is a√2, then α is equal to x≥0,y≥0x\ge 0, \quad y\ge 0 y2≤4x⇒y≤2x,y2≤12−2x⇒y≤12−2xy^{2}\le 4x \Rightarrow y\le 2\sqrt{x}, \quad y^{2}\le 12-2x \Rightarrow y\le \sqrt{12-2x} 3y+8 x≤583y+\sqrt{8}\,x\le 5\sqrt{8} Area(S)=a2⇒a=?\text{Area}(S)=a\sqrt{2} \Rightarrow a=? (1) 172,(2) 173,(3) 174,(4) 175(1)\ \tfrac{17}{2}, \quad (2)\ \tfrac{17}{3}, \quad (3)\ \tfrac{17}{4}, \quad (4)\ \tfrac{17}{5} 1. boundaries: y=2x, y=12−2x, line1.\ \text{boundaries: } y=2\sqrt{x},\ y=\sqrt{12-2x},\ \text{line} 2. which bound is active where2.\ \text{which bound is active where} 3. integrate, match a23.\ \text{integrate, match } a\sqrt{2} y=2x meets y=12−2x: 4x=12−2x⇒x=2y=2\sqrt{x} \text{ meets } y=\sqrt{12-2x}: \ 4x=12-2x \Rightarrow x=2 P=(2, 22)P=(2,\ 2\sqrt{2}) 3y+8 x=583y+\sqrt{8}\,x=5\sqrt{8} P: 62+42=102=58 ✓;y=0⇒x=5P:\ 6\sqrt{2}+4\sqrt{2}=10\sqrt{2}=5\sqrt{8}\ \checkmark; \quad y=0 \Rightarrow x=5 0≤x≤2:upper=2x0\le x\le 2: \text{upper}=2\sqrt{x} 2≤x≤5:upper=83(5−x)=223(5−x)2\le x\le 5: \text{upper}=\dfrac{\sqrt{8}}{3}(5-x)=\dfrac{2\sqrt{2}}{3}(5-x) ∫022x dx=[43x3/2]02=43⋅22=823\int_0^2 2\sqrt{x}\,dx=\left[\dfrac{4}{3}x^{3/2}\right]_0^2=\dfrac{4}{3}\cdot 2\sqrt{2}=\dfrac{8\sqrt{2}}{3} ∫25223(5−x) dx=223[5x−x22]25\int_2^5 \dfrac{2\sqrt{2}}{3}(5-x)\,dx=\dfrac{2\sqrt{2}}{3}\left[5x-\dfrac{x^2}{2}\right]_2^5 =223⋅92=32=\dfrac{2\sqrt{2}}{3}\cdot\dfrac{9}{2}=3\sqrt{2} Area=823+32=1723=a2\text{Area}=\dfrac{8\sqrt{2}}{3}+3\sqrt{2}=\dfrac{17\sqrt{2}}{3}=a\sqrt{2} ⇒a=173\Rightarrow a=\dfrac{17}{3} a=173 ⇒ option (2)\boxed{a=\dfrac{17}{3} \ \Rightarrow\ \text{option }(2)}

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