Given $\displaystyle y_n=\dfrac12(b+y_n^2)$ with $\displaystyle y_0=0.$

Show that $\displaystyle y_n$ is monotone.

I'm getting trouble to do that. It's a part of another problem which says to prove that $\displaystyle y_n$ converges, so in order to do that, I already proved that $\displaystyle y_n$ is bounded below.