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Area Under the Curves question

2023 · 15 Apr · Shift 1 · Q40
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Area Under the Curves question

2023 · 15 Apr · Shift 1 · Q40

JEE MainMathematicsArea Under the CurvesNumerical+4 / −1
If the area bounded by the curve 2y2=3x2 y^{2}=3 x2y2=3x, lines x+y=3,y=0x+y=3, y=0x+y=3,y=0 and outside the circle (x−3)2+y2=2(x-3)^{2}+y^{2}=2(x−3)2+y2=2 is A\mathrm{A}A, then 4(π+4A)4(\pi+4 A)4(π+4A) is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 42

  1. Rewrite the given curves

    The parabola is 2y2=3x  ⟹  x=23y2.2y^2=3x \implies x=\frac{2}{3}y^2.2y2=3x⟹x=32​y2.

    The line is x+y=3  ⟹  x=3−y.x+y=3 \implies x=3-y.x+y=3⟹x=3−y.

    Also, y=0y=0y=0 is the xxx-axis.

    The circle is (x−3)2+y2=2,(x-3)^2+y^2=2,(x−3)2+y2=2, centered at (3,0)(3,0)(3,0) with radius 2\sqrt{2}2​.

  2. Find the region bounded by parabola, line, and y=0y=0y=0

    The parabola and line intersect where 23y2=3−y.\frac{2}{3}y^2=3-y.32​y2=3−y. Multiply by 333: 2y2=9−3y2y^2=9-3y2y2=9−3y 2y2+3y−9=0.2y^2+3y-9=0.2y2+3y−9=0. Solving, y=−3±94.y=\frac{-3\pm 9}{4}.y=4−3±9​. So, y=32, −3.y=\frac{3}{2},\,-3.y=23​,−3. Since the region is with y=0y=0y=0, we use 0≤y≤320\le y\le \frac{3}{2}0≤y≤23​.

    For this strip,

    • left boundary: x=23y2x=\frac{2}{3}y^2x=32​y2
    • right boundary: x=3−yx=3-yx=3−y

    Hence total bounded area is ∫03/2[(3−y)−23y2]dy.\int_0^{3/2}\left[(3-y)-\frac{2}{3}y^2\right]dy.∫03/2​[(3−y)−32​y2]dy.

  3. Understand the part outside the circle

    We need the area inside the above bounded region but outside the circle.

    First check where the line and parabola meet the circle.

    Intersection of line and circle

    Put x=3−yx=3-yx=3−y into circle: ((3−y)−3)2+y2=2((3-y)-3)^2+y^2=2((3−y)−3)2+y2=2 y2+y2=2y^2+y^2=2y2+y2=2 2y2=2  ⟹  y=±1.2y^2=2 \implies y=\pm 1.2y2=2⟹y=±1. In our region, y=1y=1y=1. So point is (x,y)=(2,1).(x,y)=(2,1).(x,y)=(2,1).

    Intersection of parabola and circle

    Put x=23y2x=\frac{2}{3}y^2x=32​y2 into circle: (23y2−3)2+y2=2.\left(\frac{2}{3}y^2-3\right)^2+y^2=2.(32​y2−3)2+y2=2. Checking y=1y=1y=1, x=23,x=\frac{2}{3},x=32​, and (23−3)2+1=(−73)2+1=499+1>2,\left(\frac{2}{3}-3\right)^2+1=\left(-\frac{7}{3}\right)^2+1=\frac{49}{9}+1>2,(32​−3)2+1=(−37​)2+1=949​+1>2, so no relevant upper intersection there.

    More importantly, in the bounded region, the circle cuts off the right-hand portion near (3,0)(3,0)(3,0), and this happens for 0≤y≤10\le y\le 10≤y≤1.

    For the upper semicircle, y=2−(x−3)2  ⟺  x=3±2−y2.y=\sqrt{2-(x-3)^2} \iff x=3\pm\sqrt{2-y^2}.y=2−(x−3)2​⟺x=3±2−y2​. Since the bounded region lies to the left of x=3x=3x=3, relevant circle boundary is x=3−2−y2.x=3-\sqrt{2-y^2}.x=3−2−y2​.

    For a fixed y∈[0,1]y\in[0,1]y∈[0,1]:

    • region inside the original bounded figure runs from x=23y2x=\frac{2}{3}y^2x=32​y2 to x=3−yx=3-yx=3−y
    • inside the circle corresponds to x≥3−2−y2x\ge 3-\sqrt{2-y^2}x≥3−2−y2​ up to x=3−yx=3-yx=3−y

    Therefore, the part outside the circle is:

    • for 0≤y≤10\le y\le 10≤y≤1: from x=23y2x=\frac{2}{3}y^2x=32​y2 to x=3−2−y2x=3-\sqrt{2-y^2}x=3−2−y2​
    • for 1≤y≤321\le y\le \frac{3}{2}1≤y≤23​: whole strip from x=23y2x=\frac{2}{3}y^2x=32​y2 to x=3−yx=3-yx=3−y

    So, A=∫01[3−2−y2−23y2]dy+∫13/2[3−y−23y2]dy.A=\int_0^1\left[3-\sqrt{2-y^2}-\frac{2}{3}y^2\right]dy+\int_1^{3/2}\left[3-y-\frac{2}{3}y^2\right]dy.A=∫01​[3−2−y2​−32​y2]dy+∫13/2​[3−y−32​y2]dy.

  4. Evaluate the second integral

    I2=∫13/2(3−y−23y2)dy.I_2=\int_1^{3/2}\left(3-y-\frac{2}{3}y^2\right)dy.I2​=∫13/2​(3−y−32​y2)dy.

    Antiderivative: 3y−y22−29y3.3y-\frac{y^2}{2}-\frac{2}{9}y^3.3y−2y2​−92​y3.

    Hence I2=[3y−y22−29y3]13/2.I_2=\left[3y-\frac{y^2}{2}-\frac{2}{9}y^3\right]_1^{3/2}.I2​=[3y−2y2​−92​y3]13/2​.

    At y=32y=\frac{3}{2}y=23​:

    =\frac{9}{2}-\frac{9}{8}-\frac{3}{4} =\frac{21}{8}.$$ At $y=1$: $$3-\frac{1}{2}-\frac{2}{9}=\frac{41}{18}.$$ So, $$I_2=\frac{21}{8}-\frac{41}{18}=\frac{25}{72}.$$
  5. Evaluate the first integral

    I1=∫01(3−23y2)dy−∫012−y2 dy.I_1=\int_0^1\left(3-\frac{2}{3}y^2\right)dy-\int_0^1\sqrt{2-y^2}\,dy.I1​=∫01​(3−32​y2)dy−∫01​2−y2​dy.

    First part:

    =\left[3y-\frac{2}{9}y^3\right]_0^1 =3-\frac{2}{9}=\frac{25}{9}.$$ Now, $$\int_0^1\sqrt{2-y^2}\,dy.$$ Using the standard formula $$\int \sqrt{a^2-y^2}\,dy=\frac{y}{2}\sqrt{a^2-y^2}+\frac{a^2}{2}\sin^{-1}\left(\frac{y}{a}\right),$$ with $a=\sqrt{2}$, $$\int_0^1\sqrt{2-y^2}\,dy =\left[\frac{y}{2}\sqrt{2-y^2}+\sin^{-1}\left(\frac{y}{\sqrt2}\right)\right]_0^1.

    At y=1y=1y=1: 12⋅1+sin⁡−1(12)=12+π4.\frac{1}{2}\cdot 1+\sin^{-1}\left(\frac{1}{\sqrt2}\right)=\frac12+\frac\pi4.21​⋅1+sin−1(2​1​)=21​+4π​.

    At y=0y=0y=0: 000.

    Hence ∫012−y2 dy=12+π4.\int_0^1\sqrt{2-y^2}\,dy=\frac12+\frac\pi4.∫01​2−y2​dy=21​+4π​.

    Therefore, I1=259−(12+π4)=4118−π4.I_1=\frac{25}{9}-\left(\frac12+\frac\pi4\right)=\frac{41}{18}-\frac\pi4.I1​=925​−(21​+4π​)=1841​−4π​.

  6. Compute AAA

    A=I1+I2=(4118−π4)+2572.A=I_1+I_2=\left(\frac{41}{18}-\frac\pi4\right)+\frac{25}{72}.A=I1​+I2​=(1841​−4π​)+7225​.

    4118=16472,\frac{41}{18}=\frac{164}{72},1841​=72164​, so

    =\frac{189}{72}-\frac\pi4 =\frac{21}{8}-\frac\pi4.$$
  7. Find 4(π+4A)4(\pi+4A)4(π+4A)

    First, 4A=4(218−π4)=212−π.4A=4\left(\frac{21}{8}-\frac\pi4\right)=\frac{21}{2}-\pi.4A=4(821​−4π​)=221​−π.

    Then, π+4A=π+212−π=212.\pi+4A=\pi+\frac{21}{2}-\pi=\frac{21}{2}.π+4A=π+221​−π=221​.

    Therefore, 4(π+4A)=4⋅212=42.4(\pi+4A)=4\cdot\frac{21}{2}=42.4(π+4A)=4⋅221​=42.

  8. Compare with stored answer

    Derived answer is 424242, which matches the stored correct answer.

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