I am reading a paper on the force between hooft polyakov monopoles, but I am completely baffled by one of the 'elementary trignometric' equation they have got using an approximation. Consider a triangle say triangle ABC with angle ABC $\theta_1$, angle ACB $\theta_2$, side AB = $r_1$, side AC = $r_2$ and side BC = $s$. The author says that when A is very close to B, i.e. when $\cos{\theta_2}\approx 1$, we get $\cos{\theta_2}-1=-\frac{1}{2s^2}r_1^{2}\cos^2{\theta_1}$. Please can anyone tell me, how has this been done. I am extremely sorry if this is a silly question.