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Definite Integration question

2023 · 25 Jan · Shift 1 · Q25
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  5. /2023 · 25 Jan · Shift 1 · Q25

Definite Integration question

2023 · 25 Jan · Shift 1 · Q25

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
The minimum value of the function f(x)=∫02e∣x−t∣dtf(x) = \int\limits_0^2 {{e^{|x - t|}}dt}f(x)=0∫2​e∣x−t∣dt is :
  1. A
    2
  2. B
    2(e−1)2(e-1)2(e−1)
  3. C
    e(e−1)e(e-1)e(e−1)
  4. D
    2e−12e-12e−1
View written solutionFree

Correct answer: B

  1. We need to minimize f(x)=∫02e∣x−t∣ dt.f(x)=\int_0^2 e^{|x-t|}\,dt.f(x)=∫02​e∣x−t∣dt.

Because of the absolute value, split into cases depending on the position of xxx relative to the interval [0,2][0,2][0,2].


  1. Case 1: 0≤x≤20\le x\le 20≤x≤2

Then

  • for 0≤t≤x0\le t\le x0≤t≤x, ∣x−t∣=x−t|x-t|=x-t∣x−t∣=x−t,
  • for x≤t≤2x\le t\le 2x≤t≤2, ∣x−t∣=t−x|x-t|=t-x∣x−t∣=t−x.

So f(x)=∫0xex−t dt+∫x2et−x dt.f(x)=\int_0^x e^{x-t}\,dt+\int_x^2 e^{t-x}\,dt.f(x)=∫0x​ex−tdt+∫x2​et−xdt.

Now evaluate each part: ∫0xex−t dt=ex∫0xe−t dt=ex(1−e−x)=ex−1,\int_0^x e^{x-t}\,dt=e^x\int_0^x e^{-t}\,dt=e^x(1-e^{-x})=e^x-1,∫0x​ex−tdt=ex∫0x​e−tdt=ex(1−e−x)=ex−1,

and ∫x2et−x dt=e−x∫x2et dt=e−x(e2−ex)=e2−x−1.\int_x^2 e^{t-x}\,dt=e^{-x}\int_x^2 e^t\,dt=e^{-x}(e^2-e^x)=e^{2-x}-1.∫x2​et−xdt=e−x∫x2​etdt=e−x(e2−ex)=e2−x−1.

Hence f(x)=ex+e2−x−2.f(x)=e^x+e^{2-x}-2.f(x)=ex+e2−x−2.

To minimize this on [0,2][0,2][0,2], differentiate: f′(x)=ex−e2−x.f'(x)=e^x-e^{2-x}.f′(x)=ex−e2−x.

Set f′(x)=0f'(x)=0f′(x)=0: ex=e2−x  ⟹  x=2−x  ⟹  x=1.e^x=e^{2-x}\implies x=2-x\implies x=1.ex=e2−x⟹x=2−x⟹x=1.

Also, f′′(x)=ex+e2−x>0,f''(x)=e^x+e^{2-x}>0,f′′(x)=ex+e2−x>0, so x=1x=1x=1 gives a minimum.

Then f(1)=e+e−2=2e−2=2(e−1).f(1)=e+e-2=2e-2=2(e-1).f(1)=e+e−2=2e−2=2(e−1).


  1. Case 2: x≤0x\le 0x≤0

For all t∈[0,2]t\in[0,2]t∈[0,2], we have t≥xt\ge xt≥x, so ∣x−t∣=t−x|x-t|=t-x∣x−t∣=t−x. Thus f(x)=∫02et−x dt=e−x∫02et dt=e−x(e2−1).f(x)=\int_0^2 e^{t-x}\,dt=e^{-x}\int_0^2 e^t\,dt=e^{-x}(e^2-1).f(x)=∫02​et−xdt=e−x∫02​etdt=e−x(e2−1).

Since x≤0x\le 0x≤0, e−x≥1e^{-x}\ge 1e−x≥1, so the minimum in this region occurs at x=0x=0x=0: f(0)=e2−1.f(0)=e^2-1.f(0)=e2−1.


  1. Case 3: x≥2x\ge 2x≥2

For all t∈[0,2]t\in[0,2]t∈[0,2], we have x≥tx\ge tx≥t, so ∣x−t∣=x−t|x-t|=x-t∣x−t∣=x−t. Thus f(x)=∫02ex−t dt=ex∫02e−t dt=ex(1−e−2).f(x)=\int_0^2 e^{x-t}\,dt=e^x\int_0^2 e^{-t}\,dt=e^x(1-e^{-2}).f(x)=∫02​ex−tdt=ex∫02​e−tdt=ex(1−e−2).

Since x≥2x\ge 2x≥2, this is increasing in xxx, so the minimum in this region occurs at x=2x=2x=2: f(2)=e2−1.f(2)=e^2-1.f(2)=e2−1.


  1. Compare the candidate minima:
  • On [0,2][0,2][0,2], minimum is 2(e−1)2(e-1)2(e−1).
  • For x≤0x\le 0x≤0, minimum is e2−1e^2-1e2−1.
  • For x≥2x\ge 2x≥2, minimum is e2−1e^2-1e2−1.

Now e2−1−(2e−2)=e2−2e+1=(e−1)2>0,e^2-1-(2e-2)=e^2-2e+1=(e-1)^2>0,e2−1−(2e−2)=e2−2e+1=(e−1)2>0, so 2(e−1)<e2−1.2(e-1)<e^2-1.2(e−1)<e2−1.

Therefore the global minimum value is 2(e−1).\boxed{2(e-1)}.2(e−1)​.

So the correct option is B.

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