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Area Under the Curves question

2023 · 30 Jan · Shift 2 · Q38
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  5. /2023 · 30 Jan · Shift 2 · Q38

Area Under the Curves question

2023 · 30 Jan · Shift 2 · Q38

JEE MainMathematicsArea Under the CurvesNumerical+4 / −1
Let AAA be the area of the region {(x,y):y≥x2,y≥(1−x)2,y≤2x(1−x)}\left\{(x, y): y \geq x^2, y \geq(1-x)^2, y \leq 2 x(1-x)\right\}{(x,y):y≥x2,y≥(1−x)2,y≤2x(1−x)}. Then 540 A540 \mathrm{~A}540 A is equal to :
Numerical answer
View written solutionFree

Correct answer: 25

  1. We need the area of the region
{(x,y):y≥x2,  y≥(1−x)2,  y≤2x(1−x)}.\{(x,y): y\ge x^2,\; y\ge (1-x)^2,\; y\le 2x(1-x)\}.{(x,y):y≥x2,y≥(1−x)2,y≤2x(1−x)}.

So the region lies above both parabolas y=x2y=x^2y=x2 and y=(1−x)2y=(1-x)^2y=(1−x)2, and below the parabola

y=2x(1−x)=2x−2x2.y=2x(1-x)=2x-2x^2.y=2x(1−x)=2x−2x2.
  1. Since yyy must be above both x2x^2x2 and (1−x)2(1-x)^2(1−x)2, the effective lower boundary is
y≥max⁡{x2,(1−x)2}.y\ge \max\{x^2,(1-x)^2\}.y≥max{x2,(1−x)2}.

Thus area exists where

2x(1−x)≥max⁡{x2,(1−x)2}.2x(1-x)\ge \max\{x^2,(1-x)^2\}.2x(1−x)≥max{x2,(1−x)2}.
  1. Split according to which of x2x^2x2 and (1−x)2(1-x)^2(1−x)2 is larger.
  • For x≤12x\le \tfrac12x≤21​, we have (1−x)2≥x2(1-x)^2\ge x^2(1−x)2≥x2, so lower curve is (1−x)2(1-x)^2(1−x)2.
  • For x≥12x\ge \tfrac12x≥21​, we have x2≥(1−x)2x^2\ge (1-x)^2x2≥(1−x)2, so lower curve is x2x^2x2.

So we first find where the top curve meets these lower curves.

  1. Intersections:

(i) Solve

2x(1−x)=(1−x)2.2x(1-x)=(1-x)^2.2x(1−x)=(1−x)2.

Expanding,

2x−2x2=1−2x+x22x-2x^2=1-2x+x^22x−2x2=1−2x+x2 3x2−4x+1=03x^2-4x+1=03x2−4x+1=0 (3x−1)(x−1)=0.(3x-1)(x-1)=0.(3x−1)(x−1)=0.

So x=13x=\tfrac13x=31​ or x=1x=1x=1. In the interval x≤12x\le \tfrac12x≤21​, relevant point is

x=13.x=\tfrac13.x=31​.

(ii) Solve

2x(1−x)=x2.2x(1-x)=x^2.2x(1−x)=x2.

Then

2x−2x2=x22x-2x^2=x^22x−2x2=x2 3x2−2x=03x^2-2x=03x2−2x=0 x(3x−2)=0.x(3x-2)=0.x(3x−2)=0.

So x=0x=0x=0 or x=23x=\tfrac23x=32​. In the interval x≥12x\ge \tfrac12x≥21​, relevant point is

x=23.x=\tfrac23.x=32​.

Hence the enclosed region exists for

13≤x≤23.\frac13\le x\le \frac23.31​≤x≤32​.
  1. Therefore,
A=∫1/31/2[2x(1−x)−(1−x)2]dx+∫1/22/3[2x(1−x)−x2]dx.A=\int_{1/3}^{1/2}\left[2x(1-x)-(1-x)^2\right]dx +\int_{1/2}^{2/3}\left[2x(1-x)-x^2\right]dx.A=∫1/31/2​[2x(1−x)−(1−x)2]dx+∫1/22/3​[2x(1−x)−x2]dx.

Simplify each integrand:

For x∈[13,12]x\in[\tfrac13,\tfrac12]x∈[31​,21​],

2x(1−x)−(1−x)2=(2x−2x2)−(1−2x+x2)=4x−3x2−1.2x(1-x)-(1-x)^2=(2x-2x^2)-(1-2x+x^2)=4x-3x^2-1.2x(1−x)−(1−x)2=(2x−2x2)−(1−2x+x2)=4x−3x2−1.

For x∈[12,23]x\in[\tfrac12,\tfrac23]x∈[21​,32​],

2x(1−x)−x2=(2x−2x2)−x2=2x−3x2.2x(1-x)-x^2=(2x-2x^2)-x^2=2x-3x^2.2x(1−x)−x2=(2x−2x2)−x2=2x−3x2.

So

A=∫1/31/2(4x−3x2−1) dx+∫1/22/3(2x−3x2) dx.A=\int_{1/3}^{1/2}(4x-3x^2-1)\,dx+\int_{1/2}^{2/3}(2x-3x^2)\,dx.A=∫1/31/2​(4x−3x2−1)dx+∫1/22/3​(2x−3x2)dx.
  1. Compute the first integral:
∫(4x−3x2−1)dx=2x2−x3−x.\int(4x-3x^2-1)dx=2x^2-x^3-x.∫(4x−3x2−1)dx=2x2−x3−x.

Thus

I1=[2x2−x3−x]1/31/2.I_1=\left[2x^2-x^3-x\right]_{1/3}^{1/2}.I1​=[2x2−x3−x]1/31/2​.

At x=12x=\tfrac12x=21​:

2⋅14−18−12=12−18−12=−18.2\cdot\frac14-\frac18-\frac12=\frac12-\frac18-\frac12=-\frac18.2⋅41​−81​−21​=21​−81​−21​=−81​.

At x=13x=\tfrac13x=31​:

2⋅19−127−13=29−127−13=627−127−927=−427.2\cdot\frac19-\frac1{27}-\frac13=\frac29-\frac1{27}-\frac13 =\frac6{27}-\frac1{27}-\frac9{27}=-\frac4{27}.2⋅91​−271​−31​=92​−271​−31​=276​−271​−279​=−274​.

Hence

I1=−18+427=5216.I_1=-\frac18+\frac4{27}=\frac{5}{216}.I1​=−81​+274​=2165​.
  1. Compute the second integral:
∫(2x−3x2)dx=x2−x3.\int(2x-3x^2)dx=x^2-x^3.∫(2x−3x2)dx=x2−x3.

Thus

I2=[x2−x3]1/22/3.I_2=\left[x^2-x^3\right]_{1/2}^{2/3}.I2​=[x2−x3]1/22/3​.

At x=23x=\tfrac23x=32​:

49−827=12−827=427.\frac49-\frac8{27}=\frac{12-8}{27}=\frac4{27}.94​−278​=2712−8​=274​.

At x=12x=\tfrac12x=21​:

14−18=18.\frac14-\frac18=\frac18.41​−81​=81​.

Hence

I2=427−18=5216.I_2=\frac4{27}-\frac18=\frac5{216}.I2​=274​−81​=2165​.
  1. Therefore,
A=I1+I2=5216+5216=10216=5108.A=I_1+I_2=\frac5{216}+\frac5{216}=\frac{10}{216}=\frac5{108}.A=I1​+I2​=2165​+2165​=21610​=1085​.
  1. Now,
540A=540⋅5108=5⋅25=25.540A=540\cdot \frac5{108}=5\cdot 25=25.540A=540⋅1085​=5⋅25=25.

So,

540A=25.\boxed{540A=25}.540A=25​.
  1. Comparison with stored answer: Stored correct answer = 25, which matches our result.
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