Trying to teach myself physics and I've run into a problem I don't quite understand. "The magnitude of dA/dt can be found by the following geometrical argument. The change of A in the time interval t to Δt is" ΔA = A(t + Δt) - A(t) And then somehow it gets to |ΔA| = 2Asin(Δ[itex]\theta[/itex]/2) I just can't see how those are equal. Especially where theta/2 comes in. This comes out of Kleppner pages 25 and 26, if that helps. Supposedly there's a sketch, but I can't see one.