This mathematical formula with a big summation and the number pi:

$\sum _{n=1}^{+\mathrm{\infty}}\frac{1}{{n}^{2}}=\frac{{\pi}^{2}}{6}$ is easy to prove. And was easy to markup. I just stole the MathML directly from the MathML Core documentation.The fraction $\frac{1}{3}$ is not a decimal number (I got this example from Getting started with MathML and styled it using CSS).

I made this one: $\frac{\frac{1}{4}}{2+3}$ as a solution to the last exercise on this same page.

I'm going to need a lot of Unicode characters for this math markup, and I'll
use
Mathematical
operators and symbols in Unicode and
List of logic
symbols as primary resources (*thanks Wikipedia!*)

P ∧ Q, P ∨ Q, P → Q, P ↔ Q, ¬P → ¬Q