I am studying Fluid Mechanics and I needed a differential area element of the side or lateral surface of a frustum. This frustum is cut from a cone. In solution manual of the book I study, differential area is given for side surfaces by $$ dA=2\pi r dz$$
The geometry and the coordinate axes are given in the picture: https://imgur.com/TQMeAQs
Please ignore SAE 10W oil and remaining parts other than the frustum. They are something to do with mechanical engineering
Can you give me a hint on how to write the equation above? Do I need to consider the unfolded frustum and draw a differential curved strip element or something?