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Application of Integration question

2016 · Shift 2 · Q26
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  5. /2016 · Shift 2 · Q26

Application of Integration question

2016 · Shift 2 · Q26

JEE AdvancedMathematicsApplication of IntegrationMCQ+3 / −1
Area of the region {(x,y)∈R2:y≥∣x+3∣,5y≤x+9≤15}\left\{ {\left( {x,y} \right) \in {R^2}:y \ge \sqrt {\left| {x + 3} \right|} ,5y \le x + 9 \le 15} \right\}{(x,y)∈R2:y≥∣x+3∣​,5y≤x+9≤15} is equal to
  1. A
    16{1 \over 6}61​
  2. B
    43{4 \over 3}34​
  3. C
    32{3 \over 2}23​
  4. D
    53{5 \over 3}35​
View written solutionFree

Correct answer: C

  1. Interpret the region

We need the area of

{(x,y)∈R2:y≥∣x+3∣,  5y≤x+9≤15}.\{(x,y)\in \mathbb R^2: y\ge \sqrt{|x+3|},\; 5y\le x+9\le 15\}.{(x,y)∈R2:y≥∣x+3∣​,5y≤x+9≤15}.

The inequalities imply:

  • y≥∣x+3∣y\ge \sqrt{|x+3|}y≥∣x+3∣​
  • x+9≥5y⇒x≥5y−9x+9\ge 5y \Rightarrow x\ge 5y-9x+9≥5y⇒x≥5y−9
  • x+9≤15⇒x≤6x+9\le 15 \Rightarrow x\le 6x+9≤15⇒x≤6

So the region lies:

  • above the curve y=∣x+3∣y=\sqrt{|x+3|}y=∣x+3∣​,
  • to the right of the line x=5y−9x=5y-9x=5y−9,
  • to the left of the line x=6x=6x=6.

  1. Rewrite the curve in terms of xxx

Since y=∣x+3∣,y=\sqrt{|x+3|},y=∣x+3∣​, we have the equivalent condition y2=∣x+3∣.y^2=|x+3|.y2=∣x+3∣.

Thus:

  • for x≥−3x\ge -3x≥−3, y=x+3y=\sqrt{x+3}y=x+3​,
  • for x≤−3x\le -3x≤−3, y=−x−3y=\sqrt{-x-3}y=−x−3​.

But using horizontal strips is more convenient because of the line x=5y−9x=5y-9x=5y−9.

From y≥∣x+3∣y\ge \sqrt{|x+3|}y≥∣x+3∣​ we get y2≥∣x+3∣⇒−y2≤x+3≤y2,y^2\ge |x+3| \Rightarrow -y^2\le x+3\le y^2,y2≥∣x+3∣⇒−y2≤x+3≤y2, so −y2−3≤x≤y2−3.-y^2-3\le x\le y^2-3.−y2−3≤x≤y2−3.

Hence for a fixed yyy, the curve allows x∈[−y2−3,  y2−3].x\in[-y^2-3,\; y^2-3].x∈[−y2−3,y2−3].

The line condition gives x∈[5y−9,  6].x\in[5y-9,\; 6].x∈[5y−9,6].

Therefore, for each yyy, the allowed xxx-interval is the intersection [5y−9,6]∩[−y2−3,  y2−3].[5y-9,6]\cap[-y^2-3,\; y^2-3].[5y−9,6]∩[−y2−3,y2−3].


  1. Find the range of yyy

Since x≤6x\le 6x≤6, from the curve condition y≥∣x+3∣y\ge \sqrt{|x+3|}y≥∣x+3∣​ we must have at least some xxx satisfying ∣x+3∣≤y2|x+3|\le y^2∣x+3∣≤y2.

Also, from 5y≤x+9≤155y\le x+9\le 155y≤x+9≤15, we get 5y≤15⇒y≤3.5y\le 15 \Rightarrow y\le 3.5y≤15⇒y≤3.

And clearly y≥0y\ge 0y≥0 because ∣x+3∣≥0\sqrt{|x+3|}\ge 0∣x+3∣​≥0.

So 0≤y≤30\le y\le 30≤y≤3.

Now determine where the intersection is nonempty.

For fixed yyy, the right endpoint of curve interval is y2−3y^2-3y2−3, and the left endpoint from the line is 5y−95y-95y−9. We need 5y−9≤y2−3.5y-9\le y^2-3.5y−9≤y2−3. That is,

⇒(y−2)(y−3)≥0.\Rightarrow (y-2)(y-3)\ge 0.⇒(y−2)(y−3)≥0.

Since 0≤y≤30\le y\le 30≤y≤3, this gives 0≤y≤2ory=3.0\le y\le 2 \quad \text{or} \quad y=3.0≤y≤2ory=3. The single point y=3y=3y=3 contributes zero area, so effectively 0≤y≤20\le y\le 20≤y≤2.

Also, for 0≤y≤20\le y\le 20≤y≤2, note that y2−3≤1<6,y^2-3\le 1<6,y2−3≤1<6, so the right boundary is x=y2−3x=y^2-3x=y2−3, not x=6x=6x=6.

Thus the horizontal width is width=(y2−3)−(5y−9)=y2−5y+6=(y−2)(y−3).\text{width}= (y^2-3)-(5y-9)=y^2-5y+6=(y-2)(y-3).width=(y2−3)−(5y−9)=y2−5y+6=(y−2)(y−3).


  1. Integrate to get the area

Therefore,

A=∫02(y2−5y+6) dy.A=\int_0^2 \big(y^2-5y+6\big)\,dy.A=∫02​(y2−5y+6)dy.

Compute:

A=[y33−5y22+6y]02=83−10+12=83+2=143?A=\left[\frac{y^3}{3}-\frac{5y^2}{2}+6y\right]_0^2 =\frac{8}{3}-10+12 =\frac{8}{3}+2 =\frac{14}{3}?A=[3y3​−25y2​+6y]02​=38​−10+12=38​+2=314​?

This seems too large, so let us check carefully.

Actually,

so

A=83+2=143.A=\frac{8}{3}+2=\frac{14}{3}.A=38​+2=314​.

But this contradicts the options, so let us re-check the interval intersection.


  1. Correct interpretation of the region

The condition is y≥∣x+3∣.y\ge \sqrt{|x+3|}.y≥∣x+3∣​. Since the right-hand side is nonnegative, this means the region is above the curve. For a fixed yyy, points above the curve satisfy

so indeed

That part is correct.

Now combine with

This is

So the actual width is

For 0≤y≤20\le y\le 20≤y≤2:

  • y2−3≤1<6y^2-3\le 1<6y2−3≤1<6, so right end is y2−3y^2-3y2−3.
  • Compare 5y−95y-95y−9 and −y2−3-y^2-3−y2−3: (5y−9)−(−y2−3)=y2+5y−6=(y+6)(y−1).(5y-9)-(-y^2-3)=y^2+5y-6=(y+6)(y-1).(5y−9)−(−y2−3)=y2+5y−6=(y+6)(y−1). Hence:
    • for 0≤y≤10\le y\le 10≤y≤1, 5y−9≤−y2−35y-9\le -y^2-35y−9≤−y2−3, so left end is −y2−3-y^2-3−y2−3;
    • for 1≤y≤21\le y\le 21≤y≤2, 5y−9≥−y2−35y-9\ge -y^2-35y−9≥−y2−3, so left end is 5y−95y-95y−9.

So we must split the integral.


  1. Compute area in two parts

For 0≤y≤10\le y\le 10≤y≤1

Width:

(y2−3)−(−y2−3)=2y2.(y^2-3)-(-y^2-3)=2y^2.(y2−3)−(−y2−3)=2y2.

Area contribution:

A1=∫012y2 dy=2⋅13=23.A_1=\int_0^1 2y^2\,dy=2\cdot \frac{1}{3}=\frac{2}{3}.A1​=∫01​2y2dy=2⋅31​=32​.

For 1≤y≤21\le y\le 21≤y≤2

Width:

(y2−3)−(5y−9)=y2−5y+6.(y^2-3)-(5y-9)=y^2-5y+6.(y2−3)−(5y−9)=y2−5y+6.

Area contribution:

A2=∫12(y2−5y+6) dy.A_2=\int_1^2 (y^2-5y+6)\,dy.A2​=∫12​(y2−5y+6)dy.

Now,

A2=[y33−5y22+6y]12.A_2=\left[\frac{y^3}{3}-\frac{5y^2}{2}+6y\right]_1^2.A2​=[3y3​−25y2​+6y]12​.

At y=2y=2y=2:

83−10+12=143.\frac{8}{3}-10+12=\frac{14}{3}.38​−10+12=314​.

At y=1y=1y=1:

13−52+6=13+72=236.\frac{1}{3}-\frac{5}{2}+6=\frac{1}{3}+\frac{7}{2}=\frac{23}{6}.31​−25​+6=31​+27​=623​.

Thus,

A2=143−236=28−236=56.A_2=\frac{14}{3}-\frac{23}{6}=\frac{28-23}{6}=\frac{5}{6}.A2​=314​−623​=628−23​=65​.

Hence total area:

A=A1+A2=23+56=46+56=96=32.A=A_1+A_2=\frac{2}{3}+\frac{5}{6}=\frac{4}{6}+\frac{5}{6}=\frac{9}{6}=\frac{3}{2}.A=A1​+A2​=32​+65​=64​+65​=69​=23​.
  1. Match with options
32\boxed{\frac{3}{2}}23​​

So the correct option is C.


  1. Compare with stored correct answer

Stored correct answer: C

Our derived answer: C

They agree.

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