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Area Under the Curves question

2025 · 29 Jan · Shift 1 · Q40
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  5. /2025 · 29 Jan · Shift 1 · Q40

Area Under the Curves question

2025 · 29 Jan · Shift 1 · Q40

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
Let the area of the region (x,y):2y≤x2+3, y+∣x∣≤3, y≥∣x−1∣(x, y) : 2y \leq x^2 + 3,\ y + |x| \leq 3, \ y \geq |x - 1|(x,y):2y≤x2+3, y+∣x∣≤3, y≥∣x−1∣ be AAA. Then 6A6A6A is equal to :
  1. A
    14
  2. B
    18
  3. C
    16
  4. D
    12
View written solutionFree

Correct answer: A

We need the area of the region satisfying

2y≤x2+3,y+∣x∣≤3,y≥∣x−1∣.2y \le x^2+3, \qquad y+|x|\le 3, \qquad y\ge |x-1|.2y≤x2+3,y+∣x∣≤3,y≥∣x−1∣.

So the region is

y≤x2+32,y≤3−∣x∣,y≥∣x−1∣.y \le \frac{x^2+3}{2}, \qquad y\le 3-|x|, \qquad y\ge |x-1|.y≤2x2+3​,y≤3−∣x∣,y≥∣x−1∣.

Thus, for each xxx, the allowed yyy lies between

∣x−1∣≤y≤min⁡(x2+32, 3−∣x∣).|x-1| \le y \le \min\left(\frac{x^2+3}{2},\, 3-|x|\right).∣x−1∣≤y≤min(2x2+3​,3−∣x∣).

We now determine where this is possible.


1. Compare the two upper bounds

We compare

x2+32and3−∣x∣.\frac{x^2+3}{2} \quad \text{and} \quad 3-|x|.2x2+3​and3−∣x∣.

Solve

x2+32≤3−∣x∣.\frac{x^2+3}{2} \le 3-|x|.2x2+3​≤3−∣x∣.

This gives

x2+3≤6−2∣x∣x^2+3 \le 6-2|x|x2+3≤6−2∣x∣ x2+2∣x∣−3≤0.x^2+2|x|-3 \le 0.x2+2∣x∣−3≤0.

Let t=∣x∣≥0t=|x|\ge 0t=∣x∣≥0. Then

t2+2t−3≤0=(t+3)(t−1)≤0.t^2+2t-3\le 0 = (t+3)(t-1)\le 0.t2+2t−3≤0=(t+3)(t−1)≤0.

Hence

0≤t≤1  ⟹  ∣x∣≤1.0\le t\le 1 \implies |x|\le 1.0≤t≤1⟹∣x∣≤1.

So:

  • for ∣x∣≤1|x|\le 1∣x∣≤1, upper bound is x2+32\dfrac{x^2+3}{2}2x2+3​,
  • for ∣x∣≥1|x|\ge 1∣x∣≥1, upper bound is 3−∣x∣3-|x|3−∣x∣.

2. Find where lower bound does not exceed upper bound

We need

∣x−1∣≤min⁡(x2+32,3−∣x∣).|x-1| \le \min\left(\frac{x^2+3}{2}, 3-|x|\right).∣x−1∣≤min(2x2+3​,3−∣x∣).

We check in intervals.

Case I: x∈[−1,1]x\in[-1,1]x∈[−1,1]

Here upper bound is x2+32\dfrac{x^2+3}{2}2x2+3​. Also, since x≤1x\le 1x≤1,

∣x−1∣=1−x.|x-1|=1-x.∣x−1∣=1−x.

Need

1−x≤x2+32.1-x \le \frac{x^2+3}{2}.1−x≤2x2+3​.

Multiply by 222:

2−2x≤x2+32-2x \le x^2+32−2x≤x2+3 0≤x2+2x+1=(x+1)2,0\le x^2+2x+1=(x+1)^2,0≤x2+2x+1=(x+1)2,

which is always true. So all x∈[−1,1]x\in[-1,1]x∈[−1,1] work.

Case II: x≥1x\ge 1x≥1

Here upper bound is 3−x3-x3−x and

∣x−1∣=x−1.|x-1|=x-1.∣x−1∣=x−1.

Need

x−1≤3−x  ⟹  2x≤4  ⟹  x≤2.x-1 \le 3-x \implies 2x\le 4 \implies x\le 2.x−1≤3−x⟹2x≤4⟹x≤2.

So x∈[1,2]x\in[1,2]x∈[1,2] work.

Case III: x≤−1x\le -1x≤−1

Here upper bound is 3+x3+x3+x (since ∣x∣=−x|x|=-x∣x∣=−x), and

∣x−1∣=1−x.|x-1|=1-x.∣x−1∣=1−x.

Need

1−x≤3+x  ⟹  −2x≤2  ⟹  x≥−1.1-x \le 3+x \implies -2x\le 2 \implies x\ge -1.1−x≤3+x⟹−2x≤2⟹x≥−1.

Together with x≤−1x\le -1x≤−1, this gives only x=−1x=-1x=−1. So no interval contribution from left side beyond the endpoint.

Hence the region exists for

x∈[−1,2].x\in[-1,2].x∈[−1,2].


3. Set up area integral

So

A=∫−11(x2+32−(1−x))dx+∫12((3−x)−(x−1))dx.A=\int_{-1}^{1}\left(\frac{x^2+3}{2}-(1-x)\right)dx+\int_{1}^{2}\left((3-x)-(x-1)\right)dx.A=∫−11​(2x2+3​−(1−x))dx+∫12​((3−x)−(x−1))dx.

Simplify integrands.

For x∈[−1,1]x\in[-1,1]x∈[−1,1]:

x2+32−(1−x)=x2+3−2+2x2=(x+1)22.\frac{x^2+3}{2}-(1-x)=\frac{x^2+3-2+2x}{2}=\frac{(x+1)^2}{2}.2x2+3​−(1−x)=2x2+3−2+2x​=2(x+1)2​.

For x∈[1,2]x\in[1,2]x∈[1,2]:

(3−x)−(x−1)=4−2x.(3-x)-(x-1)=4-2x.(3−x)−(x−1)=4−2x.

Thus

A=∫−11(x+1)22 dx+∫12(4−2x) dx.A=\int_{-1}^{1}\frac{(x+1)^2}{2}\,dx+\int_{1}^{2}(4-2x)\,dx.A=∫−11​2(x+1)2​dx+∫12​(4−2x)dx.


4. Evaluate the integrals

First integral

Let u=x+1u=x+1u=x+1. When x=−1x=-1x=−1, u=0u=0u=0; when x=1x=1x=1, u=2u=2u=2.

=\frac12\left[\frac{u^3}{3}\right]_0^2 =\frac12\cdot \frac{8}{3}=\frac{4}{3}.$$ ### Second integral $$\int_1^2 (4-2x)\,dx = [4x-x^2]_1^2=(8-4)-(4-1)=4-3=1.$$ So $$A=\frac{4}{3}+1=\frac{7}{3}.$$ Therefore, $$6A=6\cdot \frac{7}{3}=14.$$ --- ## 5. Check options - A: $14$ ✅ - B: $18$ ❌ - C: $16$ ❌ - D: $12$ ❌ So the correct option is **A**.
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