MCQMediumJEE 2023Applications of Integrals (Area)

JEE Mathematics 2023 Question with Solution

The area of the region {(x,y):x2y8x2,y7}\{(x, y): x^2 \leq y \leq 8-x^2, y \leq 7\} is:

  • A

    2424

  • B

    2121

  • C

    2020

  • D

    1818

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: The region is defined by yx2y \geq x^2, y8x2y \leq 8-x^2, and y7y \leq 7.

Find: The area of the region.

From the given inequalities, the boundary curves are y=x2y = x^2, y=8x2y = 8-x^2, and the line y=7y = 7. The intersection points mentioned in the solution are (2,4)(-2,4) and (2,4)(2,4) for y=x2y=x^2 with y=8x2y=8-x^2, and (1,7)(-1,7) and (1,7)(1,7) for y=7y=7 with y=8x2y=8-x^2.

The area is set up as

Area=2(1.7+11(82x2)dx)201x2dx\text{Area} = 2 \left( 1.7 + \int_{-1}^{1} (8 - 2x^2) \, dx \right) - 2 \int_{0}^{1} x^2 \, dx

Now evaluate the first integral:

11(82x2)dx=[8x2x33]11\int_{-1}^{1} (8 - 2x^2) \, dx = \left[ 8x - \frac{2x^3}{3} \right]_{-1}^{1}

So,

=(8(1)2(1)33)(8(1)2(1)33)= \left( 8(1) - \frac{2(1)^3}{3} \right) - \left( 8(-1) - \frac{2(-1)^3}{3} \right) =(823)(8+23)=1643=443= \left( 8 - \frac{2}{3} \right) - \left( -8 + \frac{2}{3} \right) = 16 - \frac{4}{3} = \frac{44}{3}

Next evaluate the second integral:

01x2dx=[x33]01=13\int_{0}^{1} x^2 \, dx = \left[ \frac{x^3}{3} \right]_{0}^{1} = \frac{1}{3}

Substitute these values into the area expression:

Area=2(1.7+443)2×13\text{Area} = 2 \left( 1.7 + \frac{44}{3} \right) - 2 \times \frac{1}{3}

As simplified in the provided solution,

=2(7+323223)23=2(7+103)23= 2 \left( 7 + \frac{32}{3} - \frac{22}{3} \right) - \frac{2}{3} = 2 \left( 7 + \frac{10}{3} \right) - \frac{2}{3} =2(213+103)23=2×31323=62323=603=20= 2 \left( \frac{21}{3} + \frac{10}{3} \right) - \frac{2}{3} = 2 \times \frac{31}{3} - \frac{2}{3} = \frac{62}{3} - \frac{2}{3} = \frac{60}{3} = 20

Therefore, the area of the region is 2020. The correct option is C.

Using intersection points and limits

Given: The inequalities are yx2y \geq x^2, y8x2y \leq 8-x^2, and y7y \leq 7.

Find: The enclosed area.

The key idea is to identify where the upper boundary changes. The curves y=x2y=x^2 and y=8x2y=8-x^2 intersect at x=±2x=\pm 2, giving points (2,4)(-2,4) and (2,4)(2,4). The line y=7y=7 intersects y=8x2y=8-x^2 at x=±1x=\pm 1, giving points (1,7)(-1,7) and (1,7)(1,7).

The provided solution uses symmetry and writes the area in the form

Area=2(1.7+11(82x2)dx)201x2dx\text{Area} = 2 \left( 1.7 + \int_{-1}^{1} (8 - 2x^2) \, dx \right) - 2 \int_{0}^{1} x^2 \, dx

Then each integral is computed directly.

For 11(82x2)dx\int_{-1}^{1} (8-2x^2) \, dx,

[8x2x33]11=443\left[8x-\frac{2x^3}{3}\right]_{-1}^{1} = \frac{44}{3}

For 01x2dx\int_{0}^{1} x^2 \, dx,

[x33]01=13\left[\frac{x^3}{3}\right]_{0}^{1} = \frac{1}{3}

Substituting back and simplifying as shown in the solution gives

Area=20\text{Area} = 20

Hence, the correct option is C.

Common mistakes

  • Using the same upper boundary for the entire interval is incorrect because the condition y7y \leq 7 cuts off part of the parabola y=8x2y=8-x^2. First identify where y=7y=7 intersects the parabola, then split the region accordingly.

  • Ignoring intersection points such as x=±1x=\pm 1 and x=±2x=\pm 2 leads to wrong integration limits. Always solve for curve intersections before writing the area integral.

  • Integrating only 8x2x2=82x28-x^2-x^2 = 8-2x^2 without checking the extra condition y7y \leq 7 gives an overestimate. The horizontal line changes the top boundary in part of the region.

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