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Application of Integration question

2021 · Shift 1 · Q21
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  5. /2021 · Shift 1 · Q21

Application of Integration question

2021 · Shift 1 · Q21

JEE AdvancedMathematicsApplication of IntegrationMCQ+3 / −1
The area of the region {(x,y):0≤x≤94,0≤y≤1,x≥3y,x+y≥2}\left\{ {\begin{matrix} {(x,y):0 \le x \le {9 \over 4},} & {0 \le y \le 1,} & {x \ge 3y,} & {x + y \ge 2} \\ \end{matrix} } \right\}{(x,y):0≤x≤49​,​0≤y≤1,​x≥3y,​x+y≥2​} is
  1. A
    1132{{11} \over {32}}3211​
  2. B
    3596{{35} \over {96}}9635​
  3. C
    3796{{37} \over {96}}9637​
  4. D
    1332{{13} \over {32}}3213​
View written solutionFree

Correct answer: A

1. Understanding the Region

The given region is defined by the set of inequalities:

  1. 0≤x≤940 \le x \le \frac{9}{4}0≤x≤49​
  2. 0≤y≤10 \le y \le 10≤y≤1
  3. x≥3y  ⟹  y≤x3x \ge 3y \implies y \le \frac{x}{3}x≥3y⟹y≤3x​
  4. x+y≥2  ⟹  y≥2−xx + y \ge 2 \implies y \ge 2 - xx+y≥2⟹y≥2−x

We need to find the area of the region that satisfies all these conditions simultaneously.

2. Visualizing the Region and Finding Vertices

Let's sketch the boundary lines:

  • y=x/3y = x/3y=x/3 (a line through the origin)
  • y=2−xy = 2 - xy=2−x (a line with intercepts at (2,0) and (0,2))
  • x=9/4x = 9/4x=9/4 (a vertical line)
  • y=0y = 0y=0 (the x-axis)

The region is bounded from above by y=x/3y = x/3y=x/3 and from below by y=max⁡(0,2−x)y = \max(0, 2-x)y=max(0,2−x).

Let's find the key intersection points (vertices) of the boundary lines that define our area:

  • Intersection of y=x/3y = x/3y=x/3 and y=2−xy = 2-xy=2−x: x3=2−x  ⟹  x=6−3x  ⟹  4x=6  ⟹  x=32\frac{x}{3} = 2 - x \implies x = 6 - 3x \implies 4x = 6 \implies x = \frac{3}{2}3x​=2−x⟹x=6−3x⟹4x=6⟹x=23​ The corresponding y-value is y=13(32)=12y = \frac{1}{3} \left(\frac{3}{2}\right) = \frac{1}{2}y=31​(23​)=21​. So, point A is $(3/2, 1/2).

  • Intersection of y=2−xy = 2-xy=2−x and y=0y=0y=0: 0=2−x  ⟹  x=20 = 2 - x \implies x = 20=2−x⟹x=2 So, point D is $(2, 0).

The lower boundary of the region is defined by y=2−xy = 2-xy=2−x for x∈[3/2,2]x \in [3/2, 2]x∈[3/2,2] and by y=0y=0y=0 for x∈[2,9/4]x \in [2, 9/4]x∈[2,9/4]. The upper boundary is y=x/3y=x/3y=x/3. The integration will be done from x=3/2x = 3/2x=3/2 to x=9/4x = 9/4x=9/4. We need to split the integral at x=2x=2x=2 because the lower boundary function changes.

3. Setting up the Area Integral

The area A can be calculated by integrating the difference between the upper and lower boundary functions with respect to xxx. We split the integral into two parts based on the change in the lower boundary function.

A=∫3/22(yupper−ylower)dx+∫29/4(yupper−ylower)dxA = \int_{3/2}^{2} (y_{upper} - y_{lower}) dx + \int_{2}^{9/4} (y_{upper} - y_{lower}) dxA=∫3/22​(yupper​−ylower​)dx+∫29/4​(yupper​−ylower​)dx

  • For the interval [3/2,2][3/2, 2][3/2,2], yupper=x/3y_{upper} = x/3yupper​=x/3 and ylower=2−xy_{lower} = 2-xylower​=2−x.
  • For the interval [2,9/4][2, 9/4][2,9/4], yupper=x/3y_{upper} = x/3yupper​=x/3 and ylower=0y_{lower} = 0ylower​=0.

So the integral becomes: 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) dxA=∫3/22​(3x​−(2−x))dx+∫29/4​(3x​−0)dx A=∫3/22(4x3−2)dx+∫29/4x3dxA = \int_{3/2}^{2} \left( \frac{4x}{3} - 2 \right) dx + \int_{2}^{9/4} \frac{x}{3} dxA=∫3/22​(34x​−2)dx+∫29/4​3x​dx

4. Evaluating the Integrals

Part 1: ∫3/22(4x3−2)dx=[43x22−2x]3/22=[2x23−2x]3/22\int_{3/2}^{2} \left( \frac{4x}{3} - 2 \right) dx = \left[ \frac{4}{3} \frac{x^2}{2} - 2x \right]_{3/2}^{2} = \left[ \frac{2x^2}{3} - 2x \right]_{3/2}^{2}∫3/22​(34x​−2)dx=[34​2x2​−2x]3/22​=[32x2​−2x]3/22​ =(2(2)23−2(2))−(2(3/2)23−2(3/2))= \left( \frac{2(2)^2}{3} - 2(2) \right) - \left( \frac{2(3/2)^2}{3} - 2(3/2) \right)=(32(2)2​−2(2))−(32(3/2)2​−2(3/2)) =(83−4)−(2(9/4)3−3)=(8−123)−(32−3)= \left( \frac{8}{3} - 4 \right) - \left( \frac{2(9/4)}{3} - 3 \right) = \left( \frac{8-12}{3} \right) - \left( \frac{3}{2} - 3 \right)=(38​−4)−(32(9/4)​−3)=(38−12​)−(23​−3) =−43−(−32)=−43+32=−8+96=16= -\frac{4}{3} - \left( -\frac{3}{2} \right) = -\frac{4}{3} + \frac{3}{2} = \frac{-8+9}{6} = \frac{1}{6}=−34​−(−23​)=−34​+23​=6−8+9​=61​

Part 2: ∫29/4x3dx=[x26]29/4\int_{2}^{9/4} \frac{x}{3} dx = \left[ \frac{x^2}{6} \right]_{2}^{9/4}∫29/4​3x​dx=[6x2​]29/4​ =16[(94)2−(2)2]=16(8116−4)= \frac{1}{6} \left[ \left(\frac{9}{4}\right)^2 - (2)^2 \right] = \frac{1}{6} \left( \frac{81}{16} - 4 \right)=61​[(49​)2−(2)2]=61​(1681​−4) =16(81−6416)=16(1716)=1796= \frac{1}{6} \left( \frac{81 - 64}{16} \right) = \frac{1}{6} \left( \frac{17}{16} \right) = \frac{17}{96}=61​(1681−64​)=61​(1617​)=9617​

5. Calculating Total Area

Now, we add the results from the two parts: A=16+1796A = \frac{1}{6} + \frac{17}{96}A=61​+9617​ To add these fractions, we find a common denominator, which is 96. A=1imes166imes16+1796=1696+1796=3396A = \frac{1 imes 16}{6 imes 16} + \frac{17}{96} = \frac{16}{96} + \frac{17}{96} = \frac{33}{96}A=6imes161imes16​+9617​=9616​+9617​=9633​ Simplifying the fraction by dividing the numerator and denominator by 3: A=1132A = \frac{11}{32}A=3211​

6. Conclusion

The area of the given region is 1132\frac{11}{32}3211​. This corresponds to option A.

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