Hey can I get some help with this question.

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

Thanks in advance for the help.