I am a medically retired chemical engineer and I am refreshing my mathematics, learning C++ programming, and refreshing my basic engineering as a psychological form of mindfulness.
Every once in a while, I find a problem that I like a little more than others. Since I am in calculus, there are many problems that I like, in truth, but I am a slow typist and I am learning LaTeX.
Anyhow, what engineer would not like one of Sir Newton’s derivations?
 Harding, Chris. (2018, Feb. 07). First-order Linear Differential Equation and Integrating Factor. A Derivation of Newton’s Law of Cooling. scribd[com]. Available from: https://www.scribd.com/document/371027275/Calculus-Heat-Transfer-Problem ; Google URL: https://goo.gl/mx6imi