Expansion of trignometric function around infinity

Question

I need to expand $\cos[\sqrt{x^2+a^2+b x}]$ around $x=\infty$ and want to keep terms only upto $x^{-3}$. Here $a$ and $b$ are constant. Should I first expand the function $\sqrt{(x^2+a^2+b x)}$ and then write the usual cosine series? Is this the right way to approach this question? If not then what should be the right approach. Thanks