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Area Under the Curves question

2024 · 1 Feb · Shift 1 · Q40
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  5. /2024 · 1 Feb · Shift 1 · Q40

Area Under the Curves question

2024 · 1 Feb · Shift 1 · Q40

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area enclosed by the curves xy+4y=16x y+4 y=16xy+4y=16 and x+y=6x+y=6x+y=6 is equal to :
  1. A
    28−30log⁡e228-30 \log _{\mathrm{e}} 228−30loge​2
  2. B
    30−28log⁡e230-28 \log _{\mathrm{e}} 230−28loge​2
  3. C
    30−32log⁡e230-32 \log _{\mathrm{e}} 230−32loge​2
  4. D
    32−30log⁡e232-30 \log _{\mathrm{e}} 232−30loge​2
View written solutionFree

Correct answer: C

  1. Write the curves in convenient form

The given curves are:

xy+4y=16xy+4y=16xy+4y=16 x+y=6x+y=6x+y=6

From the first equation, y(x+4)=16  ⟹  y=16x+4y(x+4)=16 \implies y=\frac{16}{x+4}y(x+4)=16⟹y=x+416​

From the second equation, y=6−xy=6-xy=6−x

So we need the area enclosed between y=16x+4andy=6−x.y=\frac{16}{x+4} \quad \text{and} \quad y=6-x.y=x+416​andy=6−x.


  1. Find the points of intersection

Set the two expressions for yyy equal:

16x+4=6−x\frac{16}{x+4}=6-xx+416​=6−x

Multiply by (x+4)(x+4)(x+4):

16=(6−x)(x+4)16=(6-x)(x+4)16=(6−x)(x+4)

Expand:

16=6x+24−x2−4x16=6x+24-x^2-4x16=6x+24−x2−4x 16=2x+24−x216=2x+24-x^216=2x+24−x2

Rearrange:

x2−2x−8=0x^2-2x-8=0x2−2x−8=0

Factor:

(x−4)(x+2)=0(x-4)(x+2)=0(x−4)(x+2)=0

Hence, x=4, −2x=4,\,-2x=4,−2

Now find corresponding yyy values using y=6−xy=6-xy=6−x:

  • For x=4x=4x=4, y=2y=2y=2
  • For x=−2x=-2x=−2, y=8y=8y=8

So the curves intersect at (4,2)(4,2)(4,2) and (−2,8)(-2,8)(−2,8).


  1. Determine which curve is above the other

Take a test value between x=−2x=-2x=−2 and x=4x=4x=4, say x=0x=0x=0.

Then yline=6−0=6y_{\text{line}}=6-0=6yline​=6−0=6 ycurve=160+4=4y_{\text{curve}}=\frac{16}{0+4}=4ycurve​=0+416​=4

So on [−2,4][-2,4][−2,4], the line y=6−xy=6-xy=6−x lies above the curve y=16x+4y=\dfrac{16}{x+4}y=x+416​.

Therefore area is

A=∫−24[(6−x)−16x+4]dxA=\int_{-2}^{4}\left[(6-x)-\frac{16}{x+4}\right]dxA=∫−24​[(6−x)−x+416​]dx


  1. Evaluate the integral

A=∫−24(6−x) dx−∫−2416x+4 dxA=\int_{-2}^{4}(6-x)\,dx-\int_{-2}^{4}\frac{16}{x+4}\,dxA=∫−24​(6−x)dx−∫−24​x+416​dx

First integral:

∫(6−x)dx=6x−x22\int (6-x)dx=6x-\frac{x^2}{2}∫(6−x)dx=6x−2x2​

So, [6x−x22]−24\left[6x-\frac{x^2}{2}\right]_{-2}^{4}[6x−2x2​]−24​

At x=4x=4x=4: 24−8=1624-8=1624−8=16

At x=−2x=-2x=−2: −12−2=−14-12-2=-14−12−2=−14

Thus, 16−(−14)=3016-(-14)=3016−(−14)=30

Now second integral:

∫16x+4dx=16ln⁡∣x+4∣\int \frac{16}{x+4}dx=16\ln|x+4|∫x+416​dx=16ln∣x+4∣

So, [16ln⁡∣x+4∣]−24=16(ln⁡8−ln⁡2)=16ln⁡4=32ln⁡2\left[16\ln|x+4|\right]_{-2}^{4}=16(\ln 8-\ln 2)=16\ln 4=32\ln 2[16ln∣x+4∣]−24​=16(ln8−ln2)=16ln4=32ln2

Hence, A=30−32ln⁡2A=30-32\ln 2A=30−32ln2


  1. Match with the options

A=30−32log⁡e2A=30-32\log_e 2A=30−32loge​2

This corresponds to Option C.


  1. Compare with stored correct answer

Stored correct answer: C

Our derived answer: C

So the answer agrees with the stored correct answer.

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