Where did these symbols come from?

Question

Where did these symbols come from? Like Pi, Fee and this weird E/sideways M and the triangle.

Answer 1

They are Greek letters. The first two you mentioned are Pi ($\pi$) and Phi ($\phi$), respectively (note the spelling). What you refer to as "weird E/sideways M" is the capital letter Sigma ($\Sigma$), and the one that looks like a triangle is the capital letter Delta ($\Delta$). Here is a table of Greek letters and their approximate equivalents, which you may find interesting.

Greek Alphabet

$ \begin{array}{l|l} \text{Name} & \text{Capital} & \text{Lowercase} & \text{Equivalent}\\ \hline \text{Alpha*} & \text{A} & \alpha & \text{A} \\ \text{Beta} & \text{B} & \beta & \text{B, V} \\ \text{Gamma} & \Gamma & \gamma & \text{G} \\ \text{Delta} & \Delta & \delta & \text{D} \\ \text{Epsilon*} & \text{E} & \varepsilon,\,\epsilon & \text{E} \\ \text{Zeta} & \text{Z} & \zeta & \text{Z} \\ \text{Eta*} & \text{H} & \eta & \text{Ee} \\ \text{Theta} & \Theta & \theta,\,\vartheta & \text{Th} \\ \text{Iota*} & \text{I} & \iota & \text{I} \\ \text{Kappa} & \text{K} & \kappa & \text{K} \\ \text{Lambda} & \Lambda & \lambda & \text{L} \\ \text{Mu} & \text{M} & \mu & \text{M} \\ \text{Nu} & \text{N} & \nu & \text{N} \\ \text{Xi} & \Xi & \xi & \text{X, Ks} \\ \text{Omicron*} & \text{O} & o & \text{O} \\ \text{Pi} & \Pi & \pi,\,\varpi & \text{P} \\ \text{Rho} & \text{P} & \rho,\,\varrho & \text{R} \\ \text{Sigma} & \Sigma & \sigma,\,\varsigma & \text{S} \\ \text{Tau} & \text{T} & \tau & \text{T} \\ \text{Upsilon*} & \text{Y} & \upsilon & \text{U} \\ \text{Phi} & \Phi & \phi,\,\varphi & \text{F, Ph} \\ \text{Chi} & \text{X} & \chi & \text{Ch, Kh} \\ \text{Psi} & \Psi & \psi & \text{Ps} \\ \text{Omega*} & \Omega & \omega & \text{O} \\ \end{array} $

$\text{*Indicates a vowel}$

Answer 2

They are mostly Greek letters that have been adopted because scientists and mathematicians ran out of letters. It also probably has to do with the first real break throughs happening in Greek culture eg. Euclid, Pythagoras, Aristotle.

Answer 3

One other thing that I would like to add, is that $\Sigma$ is the symbol used in summations. If you don't understand that, just look it up on Google.