$\text{average}(x_1,\dots,x_n) = \dfrac{x_1 + \cdots + x_n}{n}.$

$\text{cross-ratio}(z_1,z_2;z_3,z_4) = \dfrac{(z_1-z_3)(z_2-z_4)}{(z_2-z_3)(z_1-z_4)}.$