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Area Under the Curves question

2023 · 1 Feb · Shift 2 · Q31
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  5. /2023 · 1 Feb · Shift 2 · Q31

Area Under the Curves question

2023 · 1 Feb · Shift 2 · Q31

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area of the region given by {(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} is :
  1. A
    16log⁡e2−14316{\log _e}2 - {{14} \over 3}16loge​2−314​
  2. B
    8log⁡e2−1338{\log _e}2 - {{13} \over 3}8loge​2−313​
  3. C
    16log⁡e2+7316{\log _e}2 + {7 \over 3}16loge​2+37​
  4. D
    8log⁡e2+768{\log _e}2 + {7 \over 6}8loge​2+67​
View written solutionFree

Correct answer: A

  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}.

The conditions mean:

  • xy≤8⇒y≤8xxy\le 8 \Rightarrow y\le \dfrac{8}{x}xy≤8⇒y≤x8​ if x>0x>0x>0.
  • Also 1≤y≤x21\le y\le x^21≤y≤x2.

Since y≥1>0y\ge 1>0y≥1>0, we must have x>0x>0x>0 for the inequality xy≤8xy\le 8xy≤8 to give a bounded upper condition on yyy.

So the region lies in the first quadrant and satisfies

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

For the region to exist, we need

min⁡(x2,8x)≥1.\min\left(x^2,\frac{8}{x}\right)\ge 1.min(x2,x8​)≥1.
  1. Find intersection of the curves

The upper boundary changes where

x2=8xx^2 = \frac{8}{x}x2=x8​

which gives

x3=8⇒x=2.x^3=8 \Rightarrow x=2.x3=8⇒x=2.

Then the corresponding yyy-value is

y=22=4.y=2^2=4.y=22=4.

So the curves y=x2y=x^2y=x2 and y=8xy=\dfrac{8}{x}y=x8​ intersect at (2,4)(2,4)(2,4).

  1. Determine the relevant xxx-intervals

From 1≤y≤x21\le y\le x^21≤y≤x2, we need

x2≥1⇒x≥1x^2\ge 1 \Rightarrow x\ge 1x2≥1⇒x≥1

(since x>0x>0x>0).

From 1≤y≤8x1\le y\le \dfrac{8}{x}1≤y≤x8​, we need

8x≥1⇒x≤8.\frac{8}{x}\ge 1 \Rightarrow x\le 8.x8​≥1⇒x≤8.

Hence the region exists for

1≤x≤8.1\le x\le 8.1≤x≤8.

Now compare the two upper bounds:

  • 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​.

Thus the area is

∫12(x2−1) dx+∫28(8x−1)dx.\int_1^2 (x^2-1)\,dx + \int_2^8 \left(\frac{8}{x}-1\right)dx.∫12​(x2−1)dx+∫28​(x8​−1)dx.
  1. Evaluate the first integral
∫12(x2−1) dx=[x33−x]12=(83−2)−(13−1).\int_1^2 (x^2-1)\,dx = \left[\frac{x^3}{3}-x\right]_1^2 = \left(\frac{8}{3}-2\right)-\left(\frac{1}{3}-1\right).∫12​(x2−1)dx=[3x3​−x]12​=(38​−2)−(31​−1).

Simplify:

83−2=23,13−1=−23.\frac{8}{3}-2 = \frac{2}{3}, \qquad \frac{1}{3}-1 = -\frac{2}{3}.38​−2=32​,31​−1=−32​.

So,

∫12(x2−1) dx=23−(−23)=43.\int_1^2 (x^2-1)\,dx = \frac{2}{3}-\left(-\frac{2}{3}\right)=\frac{4}{3}.∫12​(x2−1)dx=32​−(−32​)=34​.
  1. Evaluate the second integral
∫28(8x−1)dx=[8ln⁡x−x]28=(8ln⁡8−8)−(8ln⁡2−2).\int_2^8 \left(\frac{8}{x}-1\right)dx = \left[8\ln x - x\right]_2^8 = (8\ln 8 - 8) - (8\ln 2 - 2).∫28​(x8​−1)dx=[8lnx−x]28​=(8ln8−8)−(8ln2−2). =8(ln⁡8−ln⁡2)−6=8ln⁡(82)−6=8ln⁡4−6.= 8(\ln 8 - \ln 2) - 6 = 8\ln\left(\frac{8}{2}\right)-6 = 8\ln 4 - 6.=8(ln8−ln2)−6=8ln(28​)−6=8ln4−6.

Since ln⁡4=2ln⁡2\ln 4 = 2\ln 2ln4=2ln2,

8ln⁡4=16ln⁡2.8\ln 4 = 16\ln 2.8ln4=16ln2.

Therefore,

∫28(8x−1)dx=16ln⁡2−6.\int_2^8 \left(\frac{8}{x}-1\right)dx = 16\ln 2 - 6.∫28​(x8​−1)dx=16ln2−6.
  1. Add both parts
Area=43+(16ln⁡2−6)=16ln⁡2−143.\text{Area} = \frac{4}{3} + (16\ln 2 - 6) = 16\ln 2 - \frac{14}{3}.Area=34​+(16ln2−6)=16ln2−314​.
  1. Match with options

This is exactly

16log⁡e2−143,16\log_e 2 - \frac{14}{3},16loge​2−314​,

which corresponds to Option A.

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