yep , sorry (sen) = (sin).
Ok, the antiderivative should be
[ t + sin(t) ] from 0 to x.
Then I get [ x + sin(x) - 0 - sin(0) ] and Yep, I found my first mistake. sin(0) = 0
Btw I would like a solution without using L' Hopital rule, if there is one of course.