Thread: mathematical transition of an integral

(this posted has latex errors i dont know why)
here is a print screen of the post

in here in the end
i got an arch that startes at A and end in B
he wrote this integral

but point a and be are not values that i can put in
if my function depends on "r" then i need to change dl to be dr expression somehow
?
i think that the integral should go from 0 to pi
and dl should be d theta expression (instead of dr)
and dl =r*d(theta)
R is constant
i get

i get 2pi/R
this resolt is different from the resolt he gets
why?

He's getting 2piR/R² which is 2pi/R, there is no difference