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Definite Integration question

2024 · 4 Apr · Shift 1 · Q31
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Definite Integration question

2024 · 4 Apr · Shift 1 · Q31

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
 Let f(x)={−2,−2≤x≤0x−2,0<x≤2 and h(x)=f(∣x∣)+∣f(x)∣. Then ∫−22 h(x)dx is equal to: \text { Let } f(x)=\left\{\begin{array}{lr} -2, & -2 \leq x \leq 0 \\ x-2, & 0\lt x \leq 2 \end{array} \text { and } \mathrm{h}(x)=f(|x|)+|f(x)| \text {. Then } \int_{-2}^2 \mathrm{~h}(x) \mathrm{d} x\right. \text { is equal to: } Let f(x)={−2,x−2,​−2≤x≤00<x≤2​ and h(x)=f(∣x∣)+∣f(x)∣. Then ∫−22​ h(x)dx is equal to: 
  1. A
    2
  2. B
    6
  3. C
    4
  4. D
    1
View written solutionFree

Correct answer: A

  1. Given functions

We have

f(x)={−2,−2≤x≤0x−2,0<x≤2 f(x)= \begin{cases} -2, & -2\le x\le 0 \\ x-2, & 0<x\le 2 \end{cases}f(x)={−2,x−2,​−2≤x≤00<x≤2​

and

h(x)=f(∣x∣)+∣f(x)∣.h(x)=f(|x|)+|f(x)|.h(x)=f(∣x∣)+∣f(x)∣.

We need to evaluate

∫−22h(x) dx.\int_{-2}^{2} h(x)\,dx.∫−22​h(x)dx.
  1. Find f(∣x∣)f(|x|)f(∣x∣)

Since ∣x∣∈[0,2]|x|\in[0,2]∣x∣∈[0,2] for x∈[−2,2]x\in[-2,2]x∈[−2,2]:

  • if x=0x=0x=0, then f(∣x∣)=f(0)=−2f(|x|)=f(0)=-2f(∣x∣)=f(0)=−2,
  • if x≠0x\ne 0x=0, then ∣x∣>0|x|>0∣x∣>0, so we use the second branch: f(∣x∣)=∣x∣−2.f(|x|)=|x|-2.f(∣x∣)=∣x∣−2.

Thus, for integration purposes,

f(∣x∣)=∣x∣−2(except at x=0, where it is −2).f(|x|)=|x|-2 \quad \text{(except at }x=0\text{, where it is }-2\text{)}.f(∣x∣)=∣x∣−2(except at x=0, where it is −2).

A single point does not affect the integral.

  1. Find ∣f(x)∣|f(x)|∣f(x)∣ piecewise
  • For −2≤x≤0-2\le x\le 0−2≤x≤0:

    f(x)=−2  ⟹  ∣f(x)∣=2.f(x)=-2 \implies |f(x)|=2.f(x)=−2⟹∣f(x)∣=2.
  • For 0<x≤20<x\le 20<x≤2:

    f(x)=x−2.f(x)=x-2.f(x)=x−2.

    Since x∈(0,2]x\in(0,2]x∈(0,2], we have x−2≤0x-2\le 0x−2≤0, so

    ∣f(x)∣=∣x−2∣=2−x.|f(x)|=|x-2|=2-x.∣f(x)∣=∣x−2∣=2−x.
  1. Compute h(x)h(x)h(x) piecewise

For −2≤x≤0-2\le x\le 0−2≤x≤0

Here ∣x∣=−x|x|=-x∣x∣=−x, so

f(∣x∣)=∣x∣−2=−x−2.f(|x|)=|x|-2=-x-2.f(∣x∣)=∣x∣−2=−x−2.

Also,

∣f(x)∣=2.|f(x)|=2.∣f(x)∣=2.

Hence

h(x)=(−x−2)+2=−x.h(x)=(-x-2)+2=-x.h(x)=(−x−2)+2=−x.

For 0<x≤20<x\le 20<x≤2

Here ∣x∣=x|x|=x∣x∣=x, so

f(∣x∣)=x−2.f(|x|)=x-2.f(∣x∣)=x−2.

Also,

∣f(x)∣=2−x.|f(x)|=2-x.∣f(x)∣=2−x.

Hence

h(x)=(x−2)+(2−x)=0.h(x)=(x-2)+(2-x)=0.h(x)=(x−2)+(2−x)=0.

So,

h(x)={−x,−2≤x≤0,0,0<x≤2.h(x)= \begin{cases} -x, & -2\le x\le 0,\\ 0, & 0<x\le 2. \end{cases}h(x)={−x,0,​−2≤x≤0,0<x≤2.​

(The value at x=0x=0x=0 is irrelevant for the integral.)

  1. Integrate

Therefore,

∫−22h(x) dx=∫−20(−x) dx+∫020 dx.\int_{-2}^{2} h(x)\,dx =\int_{-2}^{0} (-x)\,dx + \int_{0}^{2} 0\,dx.∫−22​h(x)dx=∫−20​(−x)dx+∫02​0dx.

Now,

∫−20(−x) dx=[−x22]−20=0−(−42)=2.\int_{-2}^{0} (-x)\,dx =\left[-\frac{x^2}{2}\right]_{-2}^{0} =0-\left(-\frac{4}{2}\right)=2.∫−20​(−x)dx=[−2x2​]−20​=0−(−24​)=2.

And,

∫020 dx=0.\int_0^2 0\,dx=0.∫02​0dx=0.

Thus,

∫−22h(x) dx=2.\int_{-2}^{2} h(x)\,dx=2.∫−22​h(x)dx=2.
  1. Check options

The correct option is:

A: 2\boxed{\text{A: }2}A: 2​
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