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Area Under the Curves question

2022 · 28 Jul · Shift 2 · Q30
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  5. /2022 · 28 Jul · Shift 2 · Q30

Area Under the Curves question

2022 · 28 Jul · Shift 2 · Q30

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area enclosed by the curves y=log⁡e(x+e2),x=log⁡e(2y)y=\log _{e}\left(x+\mathrm{e}^{2}\right), x=\log _{e}\left(\frac{2}{y}\right)y=loge​(x+e2),x=loge​(y2​) and x=log⁡e2x=\log _{\mathrm{e}} 2x=loge​2, above the line y=1y=1y=1 is:
  1. A
    2+e−log⁡e22+\mathrm{e}-\log _{\mathrm{e}} 22+e−loge​2
  2. B
    1+e−log⁡e21+e-\log _{e} 21+e−loge​2
  3. C
    e−log⁡e2e-\log _{e} 2e−loge​2
  4. D
    1+log⁡e21+\log _{e} 21+loge​2
View written solutionFree

Correct answer: B

  1. Rewrite the given curves in convenient form

The curves are:

  • y=ln⁡(x+e2)y=\ln(x+e^2)y=ln(x+e2)
  • x=ln⁡(2y)x=\ln\left(\frac{2}{y}\right)x=ln(y2​)
  • x=ln⁡2x=\ln 2x=ln2
  • and the region is above the line y=1y=1y=1.

From x=ln⁡(2y)x=\ln\left(\frac{2}{y}\right)x=ln(y2​) we get ex=2y⇒y=2e−x.e^x=\frac{2}{y}\quad\Rightarrow\quad y=2e^{-x}.ex=y2​⇒y=2e−x.

So the relevant curves are:

  • y=ln⁡(x+e2)y=\ln(x+e^2)y=ln(x+e2)
  • y=2e−xy=2e^{-x}y=2e−x
  • x=ln⁡2x=\ln 2x=ln2
  • y=1y=1y=1

  1. Find the intersection points relevant to the enclosed region

(i) Intersection of y=2e−xy=2e^{-x}y=2e−x with y=1y=1y=1

2e−x=1⇒e−x=12⇒x=ln⁡2.2e^{-x}=1 \Rightarrow e^{-x}=\frac12 \Rightarrow x=\ln 2.2e−x=1⇒e−x=21​⇒x=ln2. So (ln⁡2,1)(\ln 2,1)(ln2,1) is one corner.

(ii) Intersection of y=ln⁡(x+e2)y=\ln(x+e^2)y=ln(x+e2) with y=1y=1y=1

ln⁡(x+e2)=1⇒x+e2=e⇒x=e−e2.\ln(x+e^2)=1 \Rightarrow x+e^2=e \Rightarrow x=e-e^2.ln(x+e2)=1⇒x+e2=e⇒x=e−e2. So (e−e2,1)(e-e^2,1)(e−e2,1) is another point.

(iii) Intersection of y=ln⁡(x+e2)y=\ln(x+e^2)y=ln(x+e2) and y=2e−xy=2e^{-x}y=2e−x

We check at x=0x=0x=0: ln⁡(0+e2)=2,2e0=2.\ln(0+e^2)=2,\qquad 2e^0=2.ln(0+e2)=2,2e0=2. So they intersect at (0,2)(0,2)(0,2).

Thus the enclosed region is bounded:

  • below by y=1y=1y=1 from x=e−e2x=e-e^2x=e−e2 to x=ln⁡2x=\ln 2x=ln2,
  • on the left-upper side by y=ln⁡(x+e2)y=\ln(x+e^2)y=ln(x+e2) from x=e−e2x=e-e^2x=e−e2 to x=0x=0x=0,
  • on the right-upper side by y=2e−xy=2e^{-x}y=2e−x from x=0x=0x=0 to x=ln⁡2x=\ln 2x=ln2.

  1. Set up the area integral

The area above y=1y=1y=1 is A=∫e−e20(ln⁡(x+e2)−1) dx+∫0ln⁡2(2e−x−1) dx.A=\int_{e-e^2}^{0}\big(\ln(x+e^2)-1\big)\,dx+\int_{0}^{\ln 2}\big(2e^{-x}-1\big)\,dx.A=∫e−e20​(ln(x+e2)−1)dx+∫0ln2​(2e−x−1)dx.


  1. Evaluate the first integral

Let I1=∫e−e20(ln⁡(x+e2)−1)dx.I_1=\int_{e-e^2}^{0}(\ln(x+e^2)-1)dx.I1​=∫e−e20​(ln(x+e2)−1)dx. Substitute u=x+e2⇒du=dx.u=x+e^2 \Rightarrow du=dx.u=x+e2⇒du=dx. When x=e−e2x=e-e^2x=e−e2, u=eu=eu=e; when x=0x=0x=0, u=e2u=e^2u=e2. So I1=∫ee2(ln⁡u−1)du.I_1=\int_e^{e^2}(\ln u-1)du.I1​=∫ee2​(lnu−1)du.

Now, ∫(ln⁡u−1)du=uln⁡u−2u.\int (\ln u-1)du = u\ln u-2u.∫(lnu−1)du=ulnu−2u. Therefore, I1=[uln⁡u−2u]ee2.I_1=\left[u\ln u-2u\right]_e^{e^2}.I1​=[ulnu−2u]ee2​.

At u=e2u=e^2u=e2: e2ln⁡(e2)−2e2=2e2−2e2=0.e^2\ln(e^2)-2e^2=2e^2-2e^2=0.e2ln(e2)−2e2=2e2−2e2=0.

At u=eu=eu=e: eln⁡e−2e=e−2e=−e.e\ln e-2e=e-2e=-e.elne−2e=e−2e=−e.

Hence, I1=0−(−e)=e.I_1=0-(-e)=e.I1​=0−(−e)=e.


  1. Evaluate the second integral

Let I2=∫0ln⁡2(2e−x−1)dx.I_2=\int_0^{\ln 2}(2e^{-x}-1)dx.I2​=∫0ln2​(2e−x−1)dx.

We have ∫(2e−x−1)dx=−2e−x−x.\int (2e^{-x}-1)dx=-2e^{-x}-x.∫(2e−x−1)dx=−2e−x−x. So I2=[−2e−x−x]0ln⁡2.I_2=\left[-2e^{-x}-x\right]_0^{\ln 2}.I2​=[−2e−x−x]0ln2​.

At x=ln⁡2x=\ln 2x=ln2: −2e−ln⁡2−ln⁡2=−2⋅12−ln⁡2=−1−ln⁡2.-2e^{-\ln 2}-\ln 2=-2\cdot \frac12-\ln 2=-1-\ln 2.−2e−ln2−ln2=−2⋅21​−ln2=−1−ln2.

At x=0x=0x=0: −2e0−0=−2.-2e^0-0=-2.−2e0−0=−2.

Thus, I2=(−1−ln⁡2)−(−2)=1−ln⁡2.I_2=(-1-\ln 2)-(-2)=1-\ln 2.I2​=(−1−ln2)−(−2)=1−ln2.


  1. Total area

A=I1+I2=e+(1−ln⁡2)=1+e−ln⁡2.A=I_1+I_2=e+(1-\ln 2)=1+e-\ln 2.A=I1​+I2​=e+(1−ln2)=1+e−ln2.


  1. Match with the options

1+e−ln⁡2\boxed{1+e-\ln 2}1+e−ln2​ which is Option B.


  1. Compare with stored correct answer

Stored correct answer: B

Our derived answer is also B, so they agree.

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