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Application of Integration question

2019 · Shift 1 · Q22
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Application of Integration question

2019 · Shift 1 · Q22

JEE AdvancedMathematicsApplication of IntegrationMCQ+3 / −1
The area of the region {(x, y) : xy ≤\le≤ 8, 1 ≤\le≤ y ≤\le≤ x2} is
  1. A
    8log⁡e2−1438{\log _e}2 - {{14} \over 3}8loge​2−314​
  2. B
    8log⁡e2−738{\log _e}2 - {{7} \over 3}8loge​2−37​
  3. C
    16log⁡e2−14316{\log _e}2 - {{14} \over 3}16loge​2−314​
  4. D
    16log⁡e2−616{\log _e}2 - 616loge​2−6
View written solutionFree

Correct answer: C

  1. Interpret the region

We need the area of {(x,y):xy≤8,  1≤y≤x2}.\{(x,y): xy \le 8,\; 1\le y \le x^2\}.{(x,y):xy≤8,1≤y≤x2}.

This means:

  • y≥1y \ge 1y≥1
  • y≤x2y \le x^2y≤x2
  • xy≤8⇒y≤8xxy \le 8 \Rightarrow y \le \dfrac{8}{x}xy≤8⇒y≤x8​ (for relevant positive xxx since y≥1y\ge 1y≥1)

So the region lies where 1≤y≤min⁡(x2,8x).1 \le y \le \min\left(x^2,\frac{8}{x}\right).1≤y≤min(x2,x8​).

Since y≥1y\ge 1y≥1, we must also have:

  • x2≥1⇒x≤−1x^2 \ge 1 \Rightarrow x\le -1x2≥1⇒x≤−1 or x≥1x\ge 1x≥1
  • and 8x≥1\dfrac{8}{x}\ge 1x8​≥1; with positive yyy, this forces x>0x>0x>0, hence x≤8x\le 8x≤8

Also, because y≤8xy\le \frac{8}{x}y≤x8​ with y≥1y\ge 1y≥1, negative xxx are not possible. Thus the region is for 1≤x≤8.1\le x\le 8.1≤x≤8.

  1. Find where the upper curves switch

The two upper bounds are y=x2andy=8x.y=x^2 \quad \text{and} \quad y=\frac{8}{x}.y=x2andy=x8​.

Their intersection is given by x2=8x⇒x3=8⇒x=2.x^2=\frac{8}{x} \Rightarrow x^3=8 \Rightarrow x=2.x2=x8​⇒x3=8⇒x=2.

So:

  • for 1≤x≤21\le x\le 21≤x≤2, we have x2≤8xx^2\le \dfrac{8}{x}x2≤x8​, so upper bound is y=x2y=x^2y=x2
  • for 2≤x≤82\le x\le 82≤x≤8, we have 8x≤x2\dfrac{8}{x}\le x^2x8​≤x2, so upper bound is y=8xy=\dfrac{8}{x}y=x8​
  1. Set up the area integral

Hence area is A=∫12(x2−1) dx+∫28(8x−1)dx.A=\int_1^2 (x^2-1)\,dx+\int_2^8\left(\frac{8}{x}-1\right)dx.A=∫12​(x2−1)dx+∫28​(x8​−1)dx.

  1. Evaluate the first integral

∫12(x2−1) dx=[x33−x]12\int_1^2 (x^2-1)\,dx=\left[\frac{x^3}{3}-x\right]_1^2∫12​(x2−1)dx=[3x3​−x]12​ =(83−2)−(13−1)=\left(\frac{8}{3}-2\right)-\left(\frac{1}{3}-1\right)=(38​−2)−(31​−1) =23−(−23)=43.=\frac{2}{3}-\left(-\frac{2}{3}\right)=\frac{4}{3}.=32​−(−32​)=34​.

  1. Evaluate the second integral

∫28(8x−1)dx=[8ln⁡x−x]28\int_2^8\left(\frac{8}{x}-1\right)dx=\left[8\ln x-x\right]_2^8∫28​(x8​−1)dx=[8lnx−x]28​ =(8ln⁡8−8)−(8ln⁡2−2)=(8\ln 8-8)-(8\ln 2-2)=(8ln8−8)−(8ln2−2) =8(ln⁡8−ln⁡2)−6=8(\ln 8-\ln 2)-6=8(ln8−ln2)−6 =8ln⁡4−6=16ln⁡2−6.=8\ln 4-6=16\ln 2-6.=8ln4−6=16ln2−6.

  1. Add both parts

A=43+(16ln⁡2−6)=16ln⁡2−143.A=\frac{4}{3}+(16\ln 2-6)=16\ln 2-\frac{14}{3}.A=34​+(16ln2−6)=16ln2−314​.

So the area is 16log⁡e2−143.\boxed{16\log_e 2-\frac{14}{3}}.16loge​2−314​​.

  1. Compare with options

This matches Option C.

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