If x=a is an approximation for the equation f(x)=0, then $\displaystyle x=a-\frac{f(a)}{f'(a)}$ is a closer approximation, establish the formula

$\displaystyle x_{r+1}=x_r (2-Nx_r)$ as a method of successive approximations to the reciprocal of N.

I don't know where to begin.

Thank you