Hello, VkL!
Abu-Khalil is correct . . . (he could have been clearer)
We have: .
Let: .
Note that: .
Substitute: .
We will integrate by parts: .
. .
We have: .
. . By parts: .
And we have: .
. . . . . . . . .
So we have: .
. . . . . . . . .
. . . . . . . . . .
Hence: .
Back-substitute: .
We have: .
. . . . . . .