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Area Under the Curves question

2022 · 25 Jul · Shift 1 · Q28
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  5. /2022 · 25 Jul · Shift 1 · Q28

Area Under the Curves question

2022 · 25 Jul · Shift 1 · Q28

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area of the region given by A={(x,y):x2≤y≤min⁡{x+2,4−3x}}A=\left\{(x, y): x^{2} \leq y \leq \min \{x+2,4-3 x\}\right\}A={(x,y):x2≤y≤min{x+2,4−3x}} is :
  1. A
    318\frac{31}{8}831​
  2. B
    176\frac{17}{6}617​
  3. C
    196\frac{19}{6}619​
  4. D
    278\frac{27}{8}827​
View written solutionFree

Correct answer: B

  1. Interpret the region

The region is

A={(x,y):x2≤y≤min⁡{x+2, 4−3x}}.A=\{(x,y): x^2 \le y \le \min\{x+2,\,4-3x\}\}.A={(x,y):x2≤y≤min{x+2,4−3x}}.

So for each fixed xxx, the upper boundary is the smaller of the two lines:

  • y=x+2y=x+2y=x+2
  • y=4−3xy=4-3xy=4−3x

and the lower boundary is the parabola

  • y=x2y=x^2y=x2.

Thus the vertical thickness is

min⁡{x+2,4−3x}−x2,\min\{x+2,4-3x\}-x^2,min{x+2,4−3x}−x2,

but only where this is nonnegative.


  1. Find which line is the minimum

We compare the two lines:

x+2=4−3xx+2=4-3xx+2=4−3x 4x=2  ⟹  x=12.4x=2 \implies x=\frac12.4x=2⟹x=21​.

Hence:

  • for x≤12x\le \frac12x≤21​, we have x+2≤4−3xx+2 \le 4-3xx+2≤4−3x, so the minimum is x+2x+2x+2;
  • for x≥12x\ge \frac12x≥21​, the minimum is 4−3x4-3x4−3x.

So the top boundary is piecewise:

y={x+2,x≤12,4−3x,x≥12.y=\begin{cases} x+2, & x\le \frac12,\\ 4-3x, & x\ge \frac12. \end{cases}y={x+2,4−3x,​x≤21​,x≥21​.​
  1. Find where the region actually exists

We need

x2≤min⁡{x+2,4−3x}.x^2 \le \min\{x+2,4-3x\}.x2≤min{x+2,4−3x}.

So solve separately.

(i) For x≤12x\le \frac12x≤21​

Need

x2≤x+2x^2 \le x+2x2≤x+2 x2−x−2≤0x^2-x-2\le 0x2−x−2≤0 (x−2)(x+1)≤0.(x-2)(x+1)\le 0.(x−2)(x+1)≤0.

This gives

−1≤x≤2.-1\le x\le 2.−1≤x≤2.

But in this case also x≤12x\le \frac12x≤21​, so we get

−1≤x≤12.-1\le x\le \frac12.−1≤x≤21​.

(ii) For x≥12x\ge \frac12x≥21​

Need

x2≤4−3xx^2 \le 4-3xx2≤4−3x x2+3x−4≤0x^2+3x-4\le 0x2+3x−4≤0 (x+4)(x−1)≤0.(x+4)(x-1)\le 0.(x+4)(x−1)≤0.

This gives

−4≤x≤1.-4\le x\le 1.−4≤x≤1.

But here also x≥12x\ge \frac12x≥21​, so we get

12≤x≤1.\frac12\le x\le 1.21​≤x≤1.

Therefore the full interval is

−1≤x≤1.-1\le x\le 1.−1≤x≤1.
  1. Set up the area integral

So the required area is

Area=∫−11/2[(x+2)−x2]dx+∫1/21[(4−3x)−x2]dx.\text{Area}=\int_{-1}^{1/2}\big[(x+2)-x^2\big]dx+\int_{1/2}^{1}\big[(4-3x)-x^2\big]dx.Area=∫−11/2​[(x+2)−x2]dx+∫1/21​[(4−3x)−x2]dx.

That is,

Area=∫−11/2(−x2+x+2) dx+∫1/21(4−3x−x2) dx.\text{Area}=\int_{-1}^{1/2}(-x^2+x+2)\,dx+\int_{1/2}^{1}(4-3x-x^2)\,dx.Area=∫−11/2​(−x2+x+2)dx+∫1/21​(4−3x−x2)dx.
  1. Evaluate the first integral
I1=∫−11/2(−x2+x+2)dx.I_1=\int_{-1}^{1/2}(-x^2+x+2)dx.I1​=∫−11/2​(−x2+x+2)dx.

An antiderivative is

−x33+x22+2x.-\frac{x^3}{3}+\frac{x^2}{2}+2x.−3x3​+2x2​+2x.

So

I1=[−x33+x22+2x]−11/2.I_1=\left[-\frac{x^3}{3}+\frac{x^2}{2}+2x\right]_{-1}^{1/2}.I1​=[−3x3​+2x2​+2x]−11/2​.

At x=12x=\frac12x=21​:

−(1/2)33+(1/2)22+2⋅12=−124+18+1=1+224=1312.-\frac{(1/2)^3}{3}+\frac{(1/2)^2}{2}+2\cdot\frac12 =-\frac{1}{24}+\frac{1}{8}+1 =1+\frac{2}{24} =\frac{13}{12}.−3(1/2)3​+2(1/2)2​+2⋅21​=−241​+81​+1=1+242​=1213​.

At x=−1x=-1x=−1:

−(−1)33+(−1)22+2(−1)=13+12−2=56−2=−76.-\frac{(-1)^3}{3}+\frac{(-1)^2}{2}+2(-1) =\frac13+\frac12-2 =\frac{5}{6}-2 =-\frac{7}{6}.−3(−1)3​+2(−1)2​+2(−1)=31​+21​−2=65​−2=−67​.

Hence

I1=1312−(−76)=1312+1412=2712=94.I_1=\frac{13}{12}-\left(-\frac{7}{6}\right) =\frac{13}{12}+\frac{14}{12} =\frac{27}{12} =\frac94.I1​=1213​−(−67​)=1213​+1214​=1227​=49​.
  1. Evaluate the second integral
I2=∫1/21(4−3x−x2)dx.I_2=\int_{1/2}^{1}(4-3x-x^2)dx.I2​=∫1/21​(4−3x−x2)dx.

An antiderivative is

4x−3x22−x33.4x-\frac{3x^2}{2}-\frac{x^3}{3}.4x−23x2​−3x3​.

So

I2=[4x−3x22−x33]1/21.I_2=\left[4x-\frac{3x^2}{2}-\frac{x^3}{3}\right]_{1/2}^{1}.I2​=[4x−23x2​−3x3​]1/21​.

At x=1x=1x=1:

4−32−13=24−9−26=136.4-\frac32-\frac13 =\frac{24-9-2}{6} =\frac{13}{6}.4−23​−31​=624−9−2​=613​.

At x=12x=\frac12x=21​:

4⋅12−3(1/2)22−(1/2)33=2−38−124=2−1024=2−512=1912.4\cdot\frac12-\frac{3(1/2)^2}{2}-\frac{(1/2)^3}{3} =2-\frac{3}{8}-\frac{1}{24} =2-\frac{10}{24} =2-\frac{5}{12} =\frac{19}{12}.4⋅21​−23(1/2)2​−3(1/2)3​=2−83​−241​=2−2410​=2−125​=1219​.

Hence

I2=136−1912=26−1912=712.I_2=\frac{13}{6}-\frac{19}{12} =\frac{26-19}{12} =\frac{7}{12}.I2​=613​−1219​=1226−19​=127​.
  1. Total area
Area=I1+I2=94+712=2712+712=3412=176.\text{Area}=I_1+I_2=\frac94+\frac{7}{12} =\frac{27}{12}+\frac{7}{12} =\frac{34}{12} =\frac{17}{6}.Area=I1​+I2​=49​+127​=1227​+127​=1234​=617​.
  1. Match with options
176\frac{17}{6}617​

corresponds to Option B.


  1. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

So they agree.

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