MCQMediumJEE 2025Applications of Integrals (Area)

JEE Mathematics 2025 Question with Solution

Let the area enclosed between the curves y=1x2|y| = 1 - x^2 and x2+y2=1x^2 + y^2 = 1 be α\alpha. If 9α=βπ+γ9\alpha = \beta\pi + \gamma; β,γ\beta, \gamma are integers, then the value of βγ|\beta - \gamma| equals:

  • A

    2727

  • B

    1818

  • C

    1515

  • D

    3333

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: The curves are y=1x2|y| = 1 - x^2 and x2+y2=1x^2 + y^2 = 1.

Find: The value of βγ|\beta - \gamma| when 9α=βπ+γ9\alpha = \beta\pi + \gamma, where α\alpha is the enclosed area.

Using symmetry, the enclosed area is taken as four times the area in the first quadrant.

α=4[Area of circle in 1st quadrant01(1x2)dx]\alpha = 4\left[\text{Area of circle in 1st quadrant} - \int_0^1 (1 - x^2) \, dx\right]

The area of the quarter circle is

π4\frac{\pi}{4}

Now evaluate the integral:

01(1x2)dx=[xx33]01=113=23\int_0^1 (1 - x^2) \, dx = \left[x - \frac{x^3}{3}\right]_0^1 = 1 - \frac{1}{3} = \frac{2}{3}

Therefore,

α=4[π423]\alpha = 4\left[\frac{\pi}{4} - \frac{2}{3}\right]α=π83\alpha = \pi - \frac{8}{3}

Now calculate 9α9\alpha:

9α=9π249\alpha = 9\pi - 24

Comparing with 9α=βπ+γ9\alpha = \beta\pi + \gamma, we get β=9\beta = 9 and γ=24\gamma = 24.

Hence,

βγ=924=33|\beta - \gamma| = |9 - 24| = 33

Therefore, the correct option is D.

Symmetry-Based Area Setup

Given: The region is enclosed between the parabola-like curve y=1x2|y| = 1 - x^2 and the circle x2+y2=1x^2 + y^2 = 1.

Find: The required integer value βγ|\beta - \gamma|.

Because the figure is symmetric about both axes, compute the area in the first quadrant and multiply by 44.

In the first quadrant, the circle contributes area π4\frac{\pi}{4}, while the curve y=1x2y = 1 - x^2 contributes the area under it from x=0x=0 to x=1x=1.

01(1x2)dx=23\int_0^1 (1 - x^2) \, dx = \frac{2}{3}

So the enclosed area is

α=4(π423)=π83\alpha = 4\left(\frac{\pi}{4} - \frac{2}{3}\right) = \pi - \frac{8}{3}

Then

9α=9π249\alpha = 9\pi - 24

Thus, from 9α=βπ+γ9\alpha = \beta\pi + \gamma, the extracted integers are β=9\beta = 9 and γ=24\gamma = 24, giving the required value as 3333.

Common mistakes

  • Taking only the first-quadrant area and forgetting to multiply by 44. This gives only one-fourth of the enclosed region. Use symmetry carefully for both the circle and the curve y=1x2|y| = 1 - x^2.

  • Using y=1x2y = 1 - x^2 without noticing the modulus in y=1x2|y| = 1 - x^2. The modulus means the curve is symmetric about the xx-axis, so both upper and lower parts must be included.

  • Making a sign error while matching 9α=βπ+γ9\alpha = \beta\pi + \gamma. From the extracted working, 9α=9π249\alpha = 9\pi - 24, so care is needed when identifying the integer term before computing βγ|\beta - \gamma|.

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