<tex> sol_1,_2 = \frac{-b +- \sqrt{delta}}{2 * a} </tex>

cele 5 reguli fundamentale la a fi un sigma
<tex> regula sigma 1: \text{sol}_{1,2} = \frac{-b \pm \sqrt{\Delta}}{2\cdot a} </tex>
<tex> regula sigma 2: f'(c)=\frac{f(a) - f(b)}{a - b} </tex>
<tex> regula sigma 3: C_n=\frac{1}{n + 1} \cdot \binom{2 \cdot n}{n} </tex>
<tex> regula sigma 4: \sum^{n}_{1}i = \frac{n \cdot (n + 1)}{2}  </tex>
<tex> regula sigma 5: \sum^{n}_{1}i^2 = \frac{n \cdot (n + 1) \cdot (2n + 1)}{6}  </tex>
<tex> regula sigma 6: |X/G| = \frac{1}{|G|} \sum_{g \in G}|X^g| </tex>
<tex> regula sigma 7: \frac{2}{\frac{1}{a} + \frac{1}{b}} \leq \sqrt{a \cdot b} \leq \frac{a + b}{2} \leq \sqrt{\frac{a^2 + b^2}{2}}</tex>
<tex> regula sigma 8: \frac{AC}{CD} = \frac{AB}{BD}, \Delta ABC \text{ cu } \angle BAD = \angle DAC  </tex>