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Definite Integration question

2019 · 12 Jan · Shift 2 · Q24
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  5. /2019 · 12 Jan · Shift 2 · Q24

Definite Integration question

2019 · 12 Jan · Shift 2 · Q24

JEE MainMathematicsDefinite IntegrationMCQ+4 / −1
The integral ∫1e{(xe)2x−(ex)x} \int\limits_1^e {\left\{ {{{\left( {{x \over e}} \right)}^{2x}} - {{\left( {{e \over x}} \right)}^x}} \right\}} \,1∫e​{(ex​)2x−(xe​)x} loge x dx is equal to :
  1. A
    −12+1e−12e2- {1 \over 2} + {1 \over e} - {1 \over {2{e^2}}}−21​+e1​−2e21​
  2. B
    32−e−12e2{3 \over 2} - e - {1 \over {2{e^2}}}23​−e−2e21​
  3. C
    12−e−1e2{1 \over 2} - e - {1 \over {{e^2}}}21​−e−e21​
  4. D
    32−1e−12x2{3 \over 2} - {1 \over e} - {1 \over {2{x^2}}}23​−e1​−2x21​
View written solutionFree

Correct answer: B

  1. Interpret the integral

We need to evaluate

I=∫1e[(xe)2x−(ex)x]ln⁡x dx.I=\int_1^e \left[\left(\frac{x}{e}\right)^{2x}-\left(\frac{e}{x}\right)^x\right]\ln x\,dx.I=∫1e​[(ex​)2x−(xe​)x]lnxdx.

Rewrite the powers in exponential form:

(xe)2x=e2x(ln⁡x−1),(ex)x=ex(1−ln⁡x).\left(\frac{x}{e}\right)^{2x}=e^{2x(\ln x-1)}, \qquad \left(\frac{e}{x}\right)^x=e^{x(1-\ln x)}.(ex​)2x=e2x(lnx−1),(xe​)x=ex(1−lnx).

But a more useful simplification is:

(xe)2x=(x2e2)x,(ex)x=(ex)x.\left(\frac{x}{e}\right)^{2x}=\left(\frac{x^2}{e^2}\right)^x, \qquad \left(\frac{e}{x}\right)^x=\left(\frac{e}{x}\right)^x.(ex​)2x=(e2x2​)x,(xe​)x=(xe​)x.
  1. Use the substitution x=etx=e^tx=et

Let

x=et⇒dx=etdt,ln⁡x=t.x=e^t \quad \Rightarrow \quad dx=e^t dt, \quad \ln x=t.x=et⇒dx=etdt,lnx=t.

When x=1x=1x=1, t=0t=0t=0; when x=ex=ex=e, t=1t=1t=1.

Now simplify each term:

(xe)2x=(ete)2et=(et−1)2et=e2et(t−1)\left(\frac{x}{e}\right)^{2x}=\left(\frac{e^t}{e}\right)^{2e^t}=\left(e^{t-1}\right)^{2e^t}=e^{2e^t(t-1)}(ex​)2x=(eet​)2et=(et−1)2et=e2et(t−1)

and

(ex)x=(eet)et=(e1−t)et=eet(1−t).\left(\frac{e}{x}\right)^x=\left(\frac{e}{e^t}\right)^{e^t}=\left(e^{1-t}\right)^{e^t}=e^{e^t(1-t)}.(xe​)x=(ete​)et=(e1−t)et=eet(1−t).

So

I=∫01[e2et(t−1)−eet(1−t)]tet dt.I=\int_0^1 \left[e^{2e^t(t-1)}-e^{e^t(1-t)}\right] t e^t\,dt.I=∫01​[e2et(t−1)−eet(1−t)]tetdt.

This form is not immediately convenient, so we instead look for derivatives of the given expressions in terms of xxx.

  1. Differentiate the key functions

Let

f(x)=(xe)2x,g(x)=(ex)x.f(x)=\left(\frac{x}{e}\right)^{2x}, \qquad g(x)=\left(\frac{e}{x}\right)^x.f(x)=(ex​)2x,g(x)=(xe​)x.

Then

ln⁡f(x)=2x(ln⁡x−1).\ln f(x)=2x(\ln x-1).lnf(x)=2x(lnx−1).

Differentiating,

f′(x)f(x)=2ln⁡x.\frac{f'(x)}{f(x)}=2\ln x.f(x)f′(x)​=2lnx.

Hence

f′(x)=2ln⁡x(xe)2x.f'(x)=2\ln x\left(\frac{x}{e}\right)^{2x}.f′(x)=2lnx(ex​)2x.

So

(xe)2xln⁡x=12f′(x).\left(\frac{x}{e}\right)^{2x}\ln x=\frac12 f'(x).(ex​)2xlnx=21​f′(x).

Similarly,

ln⁡g(x)=x(1−ln⁡x).\ln g(x)=x(1-\ln x).lng(x)=x(1−lnx).

Differentiating,

g′(x)g(x)=−ln⁡x.\frac{g'(x)}{g(x)}=-\ln x.g(x)g′(x)​=−lnx.

Thus

g′(x)=−ln⁡x(ex)x.g'(x)=-\ln x\left(\frac{e}{x}\right)^x.g′(x)=−lnx(xe​)x.

So

(ex)xln⁡x=−g′(x).\left(\frac{e}{x}\right)^x\ln x=-g'(x).(xe​)xlnx=−g′(x).
  1. Rewrite the integral using derivatives

Therefore,

I=∫1e[12f′(x)−(−g′(x))]dx=∫1e(12f′(x)+g′(x))dx.I=\int_1^e \left[\frac12 f'(x)-\left(-g'(x)\right)\right]dx =\int_1^e \left(\frac12 f'(x)+g'(x)\right)dx.I=∫1e​[21​f′(x)−(−g′(x))]dx=∫1e​(21​f′(x)+g′(x))dx.

So

I=12[f(x)]1e+[g(x)]1e.I=\frac12\big[f(x)\big]_1^e+\big[g(x)\big]_1^e.I=21​[f(x)]1e​+[g(x)]1e​.
  1. Evaluate the boundary values

For f(x)=(xe)2xf(x)=\left(\frac{x}{e}\right)^{2x}f(x)=(ex​)2x:

f(e)=(ee)2e=1,f(e)=\left(\frac{e}{e}\right)^{2e}=1,f(e)=(ee​)2e=1, f(1)=(1e)2=1e2.f(1)=\left(\frac{1}{e}\right)^2=\frac1{e^2}.f(1)=(e1​)2=e21​.

Hence

12[f(x)]1e=12(1−1e2)=12−12e2.\frac12[f(x)]_1^e=\frac12\left(1-\frac1{e^2}\right)=\frac12-\frac1{2e^2}.21​[f(x)]1e​=21​(1−e21​)=21​−2e21​.

For g(x)=(ex)xg(x)=\left(\frac{e}{x}\right)^xg(x)=(xe​)x:

g(e)=(ee)e=1,g(e)=\left(\frac{e}{e}\right)^e=1,g(e)=(ee​)e=1, g(1)=e1=e.g(1)=\frac{e}{1}=e.g(1)=1e​=e.

Thus

[g(x)]1e=1−e.[g(x)]_1^e=1-e.[g(x)]1e​=1−e.
  1. Add the results
I=(12−12e2)+(1−e)=32−e−12e2.I=\left(\frac12-\frac1{2e^2}\right)+(1-e) =\frac32-e-\frac1{2e^2}.I=(21​−2e21​)+(1−e)=23​−e−2e21​.
  1. Match with the options

This is exactly

32−e−12e2\boxed{\frac32-e-\frac1{2e^2}}23​−e−2e21​​

which corresponds to Option B.

  1. Compare with stored correct answer

Stored correct answer: B

Our derived answer: B

So they agree.

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