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Area Under the Curves question

2025 · 3 Apr · Shift 1 · Q49
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Area Under the Curves question

2025 · 3 Apr · Shift 1 · Q49

JEE MainMathematicsArea Under the CurvesNumerical+4 / −1
The area of the region bounded by the curve y=max⁡{∣x∣,x∣x−2∣}y=\max \{|x|, x|x-2|\}y=max{∣x∣,x∣x−2∣}, the xxx-axis and the lines x=−2x=-2x=−2 and x=4x=4x=4 is equal to ‾\underline{\hspace{2cm}}​
Numerical answer
View written solutionFree

Correct answer: 12

  1. We need the area under

y=max⁡{∣x∣,  x∣x−2∣}y=\max\{|x|,\; x|x-2|\}y=max{∣x∣,x∣x−2∣}

from x=−2x=-2x=−2 to x=4x=4x=4, above the xxx-axis.

So the required area is

A=∫−24max⁡{∣x∣,x∣x−2∣} dx.A=\int_{-2}^{4} \max\{|x|,x|x-2|\}\,dx.A=∫−24​max{∣x∣,x∣x−2∣}dx.

Since the expression involves absolute values, we first simplify each part on suitable intervals.


  1. Define

f1(x)=∣x∣,f2(x)=x∣x−2∣.f_1(x)=|x|, \qquad f_2(x)=x|x-2|.f1​(x)=∣x∣,f2​(x)=x∣x−2∣.

We compare f1f_1f1​ and f2f_2f2​ on intervals divided by the critical points x=0x=0x=0 and x=2x=2x=2.


  1. For x<0x<0x<0:

∣x∣=−x,∣x−2∣=2−x|x|=-x, \qquad |x-2|=2-x∣x∣=−x,∣x−2∣=2−x so

f2(x)=x(2−x)=2x−x2.f_2(x)=x(2-x)=2x-x^2.f2​(x)=x(2−x)=2x−x2.

Since x<0x<0x<0, we have 2x−x2<02x-x^2<02x−x2<0, while ∣x∣=−x>0|x|=-x>0∣x∣=−x>0. Therefore,

max⁡{∣x∣,x∣x−2∣}=∣x∣=−x(x<0).\max\{|x|,x|x-2|\}=|x|=-x \qquad (x<0).max{∣x∣,x∣x−2∣}=∣x∣=−x(x<0).

Hence on [−2,0][-2,0][−2,0],

y=−x.y=-x.y=−x.


  1. For 0≤x<20\le x<20≤x<2:

∣x∣=x,∣x−2∣=2−x|x|=x, \qquad |x-2|=2-x∣x∣=x,∣x−2∣=2−x so

f2(x)=x(2−x)=2x−x2.f_2(x)=x(2-x)=2x-x^2.f2​(x)=x(2−x)=2x−x2.

Now compare with f1(x)=xf_1(x)=xf1​(x)=x:

2x−x2≥x2x-x^2 \ge x2x−x2≥x   ⟺  x−x2≥0\iff x-x^2\ge 0⟺x−x2≥0   ⟺  x(1−x)≥0.\iff x(1-x)\ge 0.⟺x(1−x)≥0.

For 0≤x≤10\le x\le 10≤x≤1, this is true; for 1≤x<21\le x<21≤x<2, it is false.

So:

  • on [0,1][0,1][0,1], max is 2x−x22x-x^22x−x2,
  • on [1,2][1,2][1,2], max is xxx.

  1. For x≥2x\ge 2x≥2:

∣x∣=x,∣x−2∣=x−2|x|=x, \qquad |x-2|=x-2∣x∣=x,∣x−2∣=x−2 so

f2(x)=x(x−2)=x2−2x.f_2(x)=x(x-2)=x^2-2x.f2​(x)=x(x−2)=x2−2x.

Compare with f1(x)=xf_1(x)=xf1​(x)=x:

x2−2x≥xx^2-2x\ge xx2−2x≥x   ⟺  x2−3x≥0\iff x^2-3x\ge 0⟺x2−3x≥0   ⟺  x(x−3)≥0.\iff x(x-3)\ge 0.⟺x(x−3)≥0.

Since x≥2x\ge 2x≥2, this holds for x≥3x\ge 3x≥3 and fails for 2≤x≤32\le x\le 32≤x≤3.

So:

  • on [2,3][2,3][2,3], max is xxx,
  • on [3,4][3,4][3,4], max is x2−2xx^2-2xx2−2x.

  1. Therefore the function becomes
\begin{cases} -x, & -2\le x\le 0,\\[4pt] 2x-x^2, & 0\le x\le 1,\\[4pt] x, & 1\le x\le 3,\\[4pt] x^2-2x, & 3\le x\le 4. \end{cases}$$ Thus $$A=\int_{-2}^{0}(-x)\,dx+\int_{0}^{1}(2x-x^2)\,dx+\int_{1}^{3}x\,dx+\int_{3}^{4}(x^2-2x)\,dx.$$ --- 7. Compute each part: $$\int_{-2}^{0}(-x)\,dx=\left[-\frac{x^2}{2}\right]_{-2}^{0}=2.$$ $$\int_{0}^{1}(2x-x^2)\,dx=\left[x^2-\frac{x^3}{3}\right]_{0}^{1}=1-\frac13=\frac23.$$ $$\int_{1}^{3}x\,dx=\left[\frac{x^2}{2}\right]_{1}^{3}=\frac{9-1}{2}=4.$$ $$\int_{3}^{4}(x^2-2x)\,dx=\left[\frac{x^3}{3}-x^2\right]_{3}^{4} =\left(\frac{64}{3}-16\right)-\left(9-9\right) =\frac{16}{3}.$$ --- 8. Add them: $$A=2+\frac23+4+\frac{16}{3}$$ $$A=6+\frac{18}{3}=12.$$ So the required area is $$\boxed{12}.$$
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