I'm trying to calculate the curvlenght for y=ln(cosx) , . But I can not figure out what I'm doing wrong.

Using sec(x) is not allowed (because we don't learn that here in Sweden).

Using formula: where .

Using ,

and

,

changing back

Setting

But when calculating I get -0,14... The correct answer should be (according to the book) which is about 0,5