- Interpret the integral
We need to evaluate
I=∫1e[(ex)2x−(xe)x]lnxdx.
Rewrite the powers in exponential form:
(ex)2x=e2x(lnx−1),(xe)x=ex(1−lnx).
But a more useful simplification is:
(ex)2x=(e2x2)x,(xe)x=(xe)x.
- Use the substitution x=et
Let
x=et⇒dx=etdt,lnx=t.
When x=1, t=0; when x=e, t=1.
Now simplify each term:
(ex)2x=(eet)2et=(et−1)2et=e2et(t−1)
and
(xe)x=(ete)et=(e1−t)et=eet(1−t).
So
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 x.
- Differentiate the key functions
Let
f(x)=(ex)2x,g(x)=(xe)x.
Then
lnf(x)=2x(lnx−1).
Differentiating,
f(x)f′(x)=2lnx.
Hence
f′(x)=2lnx(ex)2x.
So
(ex)2xlnx=21f′(x).
Similarly,
lng(x)=x(1−lnx).
Differentiating,
g(x)g′(x)=−lnx.
Thus
g′(x)=−lnx(xe)x.
So
(xe)xlnx=−g′(x).
- Rewrite the integral using derivatives
Therefore,
I=∫1e[21f′(x)−(−g′(x))]dx=∫1e(21f′(x)+g′(x))dx.
So
I=21[f(x)]1e+[g(x)]1e.
- Evaluate the boundary values
For f(x)=(ex)2x:
f(e)=(ee)2e=1,
f(1)=(e1)2=e21.
Hence
21[f(x)]1e=21(1−e21)=21−2e21.
For g(x)=(xe)x:
g(e)=(ee)e=1,
g(1)=1e=e.
Thus
[g(x)]1e=1−e.
- Add the results
I=(21−2e21)+(1−e)=23−e−2e21.
- Match with the options
This is exactly
23−e−2e21
which corresponds to Option B.
- Compare with stored correct answer
Stored correct answer: B
Our derived answer: B
So they agree.