MCQMediumJEE 2023Applications of Integrals (Area)

JEE Mathematics 2023 Question with Solution

The area bounded by the curves y=x1+x2y = |x - 1| + |x - 2| and y=3y = 3 is equal to:

  • A

    55

  • B

    44

  • C

    66

  • D

    33

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: y=x1+x2y = |x - 1| + |x - 2| and y=3y = 3.

Find: The area bounded by these curves.

Break the absolute value expression into intervals:

x1+x2={(1x)+(2x)=32x,x<1(x1)+(2x)=1,1x2(x1)+(x2)=2x3,x>2|x-1|+|x-2|= \begin{cases} (1-x)+(2-x)=3-2x, & x<1 \\ (x-1)+(2-x)=1, & 1\le x\le 2 \\ (x-1)+(x-2)=2x-3, & x>2 \end{cases}

Now find the points of intersection with y=3y=3.

For x<1x<1:

32x=3x=03-2x=3 \Rightarrow x=0

For 1x21\le x\le 2, the curve is y=1y=1, so it does not intersect y=3y=3 inside this interval.

For x>2x>2:

2x3=32x=6x=32x-3=3 \Rightarrow 2x=6 \Rightarrow x=3

So the bounded region lies from x=0x=0 to x=3x=3.

Graph of the absolute value curve with a flat segment from x equals 1 to x equals 2 at y equals 1, intersected by the horizontal line y equals 3, shading the bounded region from x equals 0 to x equals 3.

Compute the area between the line and the curve:

Area=01[3(32x)]dx+12(31)dx+23[3(2x3)]dx\text{Area}=\int_0^1 \left[3-(3-2x)\right]dx+\int_1^2 \left(3-1\right)dx+\int_2^3 \left[3-(2x-3)\right]dx

Simplifying each part:

Area=012xdx+122dx+23(62x)dx\text{Area}=\int_0^1 2x\,dx+\int_1^2 2\,dx+\int_2^3 (6-2x)\,dx

Evaluate:

012xdx=[x2]01=1\int_0^1 2x\,dx = \left[x^2\right]_0^1 = 1 122dx=2\int_1^2 2\,dx = 2 23(62x)dx=[6xx2]23=1\int_2^3 (6-2x)\,dx = \left[6x-x^2\right]_2^3 = 1

Therefore,

Area=1+2+1=4\text{Area}=1+2+1=4

So, the area bounded by the curves is 44 square units. The correct option is B.

Using symmetry of the graph

Given: y=x1+x2y = |x - 1| + |x - 2| and y=3y = 3.

Find: The bounded area.

The graph has a horizontal segment y=1y=1 from x=1x=1 to x=2x=2, and straight-line portions on both sides. The intersections with y=3y=3 occur at x=0x=0 and x=3x=3.

So the shaded region can be seen as a rectangle from x=1x=1 to x=2x=2 of height 31=23-1=2, together with two congruent right triangles on the left and right.

Rectangle area:

1×2=21\times 2=2

Each triangle has base 11 and height 22, so each area is:

12×1×2=1\frac{1}{2}\times 1 \times 2 = 1

Hence total area is:

2+1+1=42+1+1=4

Therefore, the bounded area is 44 square units, so the correct option is B.

Common mistakes

  • Students often do not split x1+x2|x-1|+|x-2| into intervals at x=1x=1 and x=2x=2. This is wrong because absolute value expressions change form at those points. First write the piecewise form, then integrate on each interval.

  • Some students assume the middle part of the graph is curved. This is incorrect because for 1x21\le x\le 2, x1+x2=(x1)+(2x)=1|x-1|+|x-2|=(x-1)+(2-x)=1, which is a horizontal line. Use this constant segment to compute the central rectangular area.

  • A common error is taking the integral of the curve itself instead of the area between the line and the curve. The required area is found using upper function minus lower function, here 3(x1+x2)3-\left(|x-1|+|x-2|\right) over the bounded interval.

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