$ax^2 + bx + c = 0, \quad D = b^2 - 4ac, \quad x_{1,2} = \frac{-b \pm \sqrt{D}}{2a}$

$(a+b)^2 = a^2 + 2ab + b^2$

$a^2 - b^2 = (a-b)(a+b)$

$a^n \cdot a^m = a^{n+m}, \quad \frac{a^n}{a^m} = a^{n-m}, \quad (a^n)^m = a^{n \cdot m}$

$a_n = a_1 + d(n-1), \quad S_n = \frac{a_1 + a_n}{2} \cdot n$