ধরি, $\theta = \frac{1}{2} \cos^{-1} \frac{9}{41}$। $$ \begin{aligned} \Rightarrow 2\theta &= \cos^{-1} \frac{9}{41} \\ \Rightarrow \cos 2\theta &= \frac{9}{41} \\ \Rightarrow \frac{1 - \tan^2 \theta}{1 + \tan^2 \theta} &= \frac{9}{41} \end{aligned} $$ যোজন-বিয়োজন করে পাই, $$ \begin{aligned} \frac{1 - \tan^2 \theta + 1 + \tan^2 \theta}{1 - \tan^2 \theta - 1 - \tan^2 \theta} &= \frac{9+41}{9-41} \\ \Rightarrow \frac{2}{-2 \tan^2 \theta} &= \frac{50}{-32} \\ \Rightarrow \tan^2 \theta &= \frac{32}{50} = \frac{16}{25} \\ \Rightarrow \tan \theta &= \frac{4}{5} \\ \Rightarrow \theta &= \tan^{-1} \frac{4}{5} \end{aligned} $$