Let $x_0,x_1\text{and}{x}_2$  be three distinct real numbers.  Define three polynomials $l_0\left(x\right),l_1\left(x\right),l_2\left(x\right)$  and $l\left(x\right)$  as follows :

$l_0\left(x\right)=\frac{\left(x-x_1\right)\left(x-x_2\right)}{\left(x_0-x_1\right)\left(x_0-x_2\right)},l_1\left(x\right)=\frac{\left(x-x_0\right)\left(x-x_2\right)}{\left(x_1-x_0\right)\left(x_1-x_2\right)}$  and $l_2\left(x\right)=\frac{\left(x-x_0\right)\left(x-x_1\right)}{\left(x_2-x_0\right)\left(x_2-x_1\right)}$ , $l\left(x\right)=\left(x-x_0\right)\left(x-x_1\right)\left(x-x_2\right)$