int_0^1(x^7-1)/lnxdx∫10x7−1lnxdx = ?
1 Answer
Explanation:
We want to solve
I=int_0^1(x^7-1)/ln(x)dxI=∫10x7−1ln(x)dx
The indefinite integral involves the exponential- and logarithmic integral, so we will take a different approach
This will be solved by differentiation under the integral sign
Introduce a parameter
I(a)=int_0^1(x^a-1)/ln(x)dxI(a)=∫10xa−1ln(x)dx
Notice, we seek
Differentiate both sides with respect to
I'(a)=int_0^1d/(da)((x^a-1)/ln(x))dx
color(white)(I'(a))=int_0^1x^adx
color(white)(I'(a))=[x^(a+1)/(a+1)]_0^1
color(white)(I'(a))=1/(a+1)
Integrate both sides with respect to
I(a)=int1/(a+1)da=ln(a+1)+C
But
0=ln(0+1)+C=>C=0
Thus
I(a)=ln(a+1)
For
I(7)=ln(7+1)=ln(8)
Or equivalent
I=int_0^1(x^7-1)/ln(x)dx=ln(8)