1. The problem statement, all variables and given/known data 2. Relevant equations Fluid Mechanics τ =μ (du/dy) 3. The attempt at a solution I got far enough to write down dM = μ (Ωr/tanθ) dA from just substitutions, easy enough. I get confused when I'm solving for the differential surface area. I somehow need the dA for a cone. For a circle it's easy enough. A=πr^2 so then dA = 2πr dr. But how do I go about getting this for a cone? the book lists it as (2πr/cosθ) dr, without any explanation, of course.