int_0^1(x^7-1)/lnxdx10x71lnxdx = ?

1 Answer
May 4, 2018

I=ln(8)I=ln(8)

Explanation:

We want to solve

I=int_0^1(x^7-1)/ln(x)dxI=10x71ln(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 color(red)(aa, and consider

I(a)=int_0^1(x^a-1)/ln(x)dxI(a)=10xa1ln(x)dx

Notice, we seek color(blue)(I(7)I(7), and moreover that color(blue)(I(0)=0I(0)=0

Differentiate both sides with respect to color(red)(aa

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 color(red)(a

I(a)=int1/(a+1)da=ln(a+1)+C

But color(blue)(I(0)=0, so the constant can evaluated

0=ln(0+1)+C=>C=0

Thus

I(a)=ln(a+1)

For color(red)(a=7

I(7)=ln(7+1)=ln(8)

Or equivalent

I=int_0^1(x^7-1)/ln(x)dx=ln(8)