MCQMediumJEE 2026Applications of Integrals (Area)

JEE Mathematics 2026 Question with Solution

The area of the region R={(x,y):xy8,  1yx2,  x0}R=\{(x,y): xy\le 8,\; 1\le y\le x^2,\; x\ge 0\} is:

  • A

    23(20loge2+9)\dfrac{2}{3}(20\log_e 2+9)

  • B

    13(40loge2+27)\dfrac{1}{3}(40\log_e 2+27)

  • C

    13(49loge215)\dfrac{1}{3}(49\log_e 2-15)

  • D

    23(24loge27)\dfrac{2}{3}(24\log_e 2-7)

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: The region is R={(x,y):xy8,  1yx2,  x0}R=\{(x,y): xy\le 8,\; 1\le y\le x^2,\; x\ge 0\}.

Find: The area of the region.

The region is bounded by the curves y=1y=1, y=x2y=x^2, and y=8xy=\dfrac{8}{x}. The upper boundary changes where x2=8xx^2=\dfrac{8}{x}.

Find the point of intersection:

x2=8xx3=8x=2x^2=\frac{8}{x}\Rightarrow x^3=8 \Rightarrow x=2

Set up the integrals:

Area=12(x21)dx+28(8x1)dx\text{Area}=\int_1^2 (x^2-1)\,dx+\int_2^8\left(\frac{8}{x}-1\right)\,dx

For 1x21\le x\le 2, the upper curve is y=x2y=x^2. For 2x82\le x\le 8, the upper curve is y=8xy=\dfrac{8}{x}.

Evaluate the first integral:

12(x21)dx=[x33x]12=43\int_1^2(x^2-1)\,dx=\left[\frac{x^3}{3}-x\right]_1^2=\frac{4}{3}

Evaluate the second integral:

28(8x1)dx=[8lnxx]28=16ln26\int_2^8\left(\frac{8}{x}-1\right)\,dx=\left[8\ln x-x\right]_2^8=16\ln 2-6

Add the results:

Area=16ln2143=23(24ln27)\text{Area}=16\ln 2-\frac{14}{3}=\frac{2}{3}(24\ln 2-7)

Therefore, the correct option is D.

Why the integral must be split

Given: The same region satisfies xy8xy\le 8, 1yx21\le y\le x^2, and x0x\ge 0.

Find: Why two separate integrals are required.

Since xy8xy\le 8 and x>0x>0 in the region, this becomes y8xy\le \dfrac{8}{x}. Together with 1yx21\le y\le x^2, the actual upper boundary is the smaller of x2x^2 and 8x\dfrac{8}{x}.

The switch occurs when the two upper candidates are equal:

x2=8xx=2x^2=\frac{8}{x}\Rightarrow x=2

Also, from 1yx21\le y\le x^2, we need x21x^2\ge 1, so x1x\ge 1. From 1y8x1\le y\le \dfrac{8}{x}, we need 8x1\dfrac{8}{x}\ge 1, so x8x\le 8. Hence the interval is 1x81\le x\le 8.

Thus the area is computed as

12(x21)dx+28(8x1)dx\int_1^2 (x^2-1)\,dx+\int_2^8\left(\frac{8}{x}-1\right)\,dx

which evaluates to

43+16ln26=16ln2143=23(24ln27)\frac{4}{3}+16\ln 2-6=16\ln 2-\frac{14}{3}=\frac{2}{3}(24\ln 2-7)

So the correct option is D.

Common mistakes

  • Using a single integral over the whole interval is incorrect because the upper boundary changes from y=x2y=x^2 to y=8xy=\dfrac{8}{x} at x=2x=2. Always find the intersection point first and split the area accordingly.

  • Treating xy8xy\le 8 as only the curve xy=8xy=8 is wrong. The inequality means the region lies below y=8xy=\dfrac{8}{x} for x>0x>0, so it acts as a boundary condition on the admissible area.

  • Missing the xx-limits 11 and 88 leads to an incomplete region. These come from requiring the lower boundary y=1y=1 to stay below the upper bounds, giving x21x^2\ge 1 and 8x1\dfrac{8}{x}\ge 1.

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