## Interpolatory Quadrature Formula

Hey can I get some help with this question.

Show that if $I_n$ is an interpolatory quadrature formula then the sum
$\sum_{i=0}^{n} \sigma_i$
of the weights of $I_n$ is equal to 1 if the degree of exactness of $I_n$ is greater than zero.

Thanks in advance for the help.