Limits Continuity And Differentiability

If the function $f(x)=\left\{\begin{array}{cl}\frac{\cos a x-\cos b x}{\cos c x-\cos b x} & , \text { if } x \neq 0 \\ -1 & , \text { if } x=0\end{array}\right.$ is continuous at $x=0$, then $\mathrm{a}^2, \mathrm{~b}^2, \mathrm{c}^2$ are in