Partial differential equation - Wikipedia
https://en.wikipedia.org/wiki/Partial_differential_equationWhen writing PDEs, it is common to denote partial derivatives using subscripts. For example: The Greek letter Δ denotes the Laplace operator; if u is a function of n variables, then A PDE is called linear if it is linear in the unknown and its derivatives. For example, for a function u of x and y, a second order linear PDE is of the form Nearest to linear PDEs are semilinear PDEs, where the highest order derivatives appear only as li…
Ellipse - Wikipedia
en.wikipedia.org › wiki › Ellipse1) except for the section on the area enclosed by a tilted ellipse, where the generalized form of Eq.(1) will be given. Area The area A ellipse {\displaystyle A_{\text{ellipse}}} enclosed by an ellipse is: A ellipse = π a b {\displaystyle A_{\text{ellipse}}=\pi \;a\,b} (2) where a {\displaystyle a} and b {\displaystyle b} are the lengths of the semi-major and semi-minor axes, respectively ...