MCQMediumJEE 2025Applications of Integrals (Area)

JEE Mathematics 2025 Question with Solution

If the area of the region {(x,y):1+x2ymin(x+7,113x)}\{(x, y) : 1 + x^2 \leq y \leq \min(x + 7, 11 - 3x)\} is AA, then 3A3A is equal to:

  • A

    5050

  • B

    4949

  • C

    4646

  • D

    4747

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The region is defined by 1+x2ymin(x+7,113x)1 + x^2 \leq y \leq \min(x + 7, 11 - 3x).

Find: The value of 3A3A, where AA is the area of the region.

First identify which line gives the upper boundary. The two lines intersect at

x+7=113xx + 7 = 11 - 3x

so

4x=44x = 4

and hence

x=1.x = 1.

Therefore, for x<1x < 1 the minimum is x+7x + 7, and for x>1x > 1 the minimum is 113x11 - 3x.

Now find where the parabola meets each line in the relevant interval.

With y=x+7y = x + 7,

1+x2=x+71 + x^2 = x + 7 x2x6=0x^2 - x - 6 = 0 (x3)(x+2)=0(x - 3)(x + 2) = 0

so the roots are x=3x = 3 and x=2x = -2. Since this part uses x<1x < 1, the valid point is x=2x = -2.

With y=113xy = 11 - 3x,

1+x2=113x1 + x^2 = 11 - 3x x2+3x10=0x^2 + 3x - 10 = 0 (x+5)(x2)=0(x + 5)(x - 2) = 0

so the roots are x=5x = -5 and x=2x = 2. Since this part uses x>1x > 1, the valid point is x=2x = 2.

Hence the area must be split into two parts:

A=A1+A2A = A_1 + A_2

where

A1=21[(x+7)(1+x2)]dxA_1 = \int_{-2}^{1} \left[(x + 7) - (1 + x^2)\right] \, dx

and

A2=12[(113x)(1+x2)]dx.A_2 = \int_{1}^{2} \left[(11 - 3x) - (1 + x^2)\right] \, dx.

For the first part,

A1=21(x+6x2)dxA_1 = \int_{-2}^{1} (x + 6 - x^2) \, dx =[x22+6xx33]21.= \left[ \frac{x^2}{2} + 6x - \frac{x^3}{3} \right]_{-2}^{1}.

Now,

(12+613)=376\left( \frac{1}{2} + 6 - \frac{1}{3} \right) = \frac{37}{6}

and

(212+83)=223.\left( 2 - 12 + \frac{8}{3} \right) = -\frac{22}{3}.

Therefore,

A1=376(223)=376+446=816=272.A_1 = \frac{37}{6} - \left(-\frac{22}{3}\right) = \frac{37}{6} + \frac{44}{6} = \frac{81}{6} = \frac{27}{2}.

For the second part,

A2=12(103xx2)dxA_2 = \int_{1}^{2} (10 - 3x - x^2) \, dx =[10x3x22x33]12.= \left[ 10x - \frac{3x^2}{2} - \frac{x^3}{3} \right]_{1}^{2}.

Now,

20683=34320 - 6 - \frac{8}{3} = \frac{34}{3}

and

103213=496.10 - \frac{3}{2} - \frac{1}{3} = \frac{49}{6}.

Therefore,

A2=343496=686496=196.A_2 = \frac{34}{3} - \frac{49}{6} = \frac{68}{6} - \frac{49}{6} = \frac{19}{6}.

So the total area is

A=272+196=816+196=1006=503.A = \frac{27}{2} + \frac{19}{6} = \frac{81}{6} + \frac{19}{6} = \frac{100}{6} = \frac{50}{3}.

Hence,

3A=3×503=50.3A = 3 \times \frac{50}{3} = 50.

Therefore, the correct option is A.

The first solution shown in the source contains an arithmetic inconsistency; the second solution correctly gives 3A=503A = 50.

Conclusion: The value of 3A3A is 5050.

Piecewise Upper Boundary Interpretation

Given: yy lies above the parabola y=1+x2y = 1 + x^2 and below the function min(x+7,113x)\min(x + 7, 11 - 3x).

Find: The required area expression and then compute 3A3A.

Because the upper boundary is a minimum of two lines, we must use the lower of the two lines at each value of xx. Their intersection occurs at x=1x = 1, so the upper boundary changes there. This is why the area is not found using a single integral over the whole interval.

The admissible region starts where the parabola first meets the active upper boundary on the left, namely at x=2x = -2, and ends where it meets the active upper boundary on the right, namely at x=2x = 2. Thus the region exists over

2x2-2 \leq x \leq 2

but with a change of upper curve at

x=1.x = 1.

Therefore,

A=21[(x+7)(1+x2)]dx+12[(113x)(1+x2)]dx.A = \int_{-2}^{1} \bigl[(x+7) - (1+x^2)\bigr] \, dx + \int_{1}^{2} \bigl[(11-3x) - (1+x^2)\bigr] \, dx.

Evaluating gives

A=272+196=503.A = \frac{27}{2} + \frac{19}{6} = \frac{50}{3}.

Hence,

3A=50.3A = 50.

So the correct option is A.

Common mistakes

  • Using a single upper boundary for the entire interval is incorrect because min(x+7,113x)\min(x+7, 11-3x) changes at x=1x=1. First compare the two lines, find where they intersect, and then split the integral at that point.

  • Taking both intersection points from each quadratic without checking the relevant interval is wrong. For x+7x+7, only the root with x<1x<1 is valid, and for 113x11-3x, only the root with x>1x>1 is valid.

  • Subtracting in the wrong order gives a negative area. The integrand must always be upper curve minus lower curve, namely the active line minus 1+x21+x^2.

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