See image attached. (I've had a google but can't find anything). I am trying to understand the expression : Rdθ.2∏Rsinθ Here are my thoughts so far: Rdθ is the width of a strip, θ being the variable changing/to integrate over, giving arise to the elements. 2∏Rsinθ must then be the length. However I dont understand this expression. I think I understand Rdθ - it is the arc length. Here are my length thoughts: From the triangle sin θ =(l/2)/R, where l is the length we are after, and l/2, because this triangle only gives half of the length we are after. Ofc this must be wrong, as I have a ∏ unexplained. Many thanks to anyone who can help shed some light on this, greatly appreciated !