 Hello and welcome to this session. Let us understand the following problem today. Find the value of cos inverse of cos 13 pi by 6. Now let us write the solution. Cos inverse of cos of 13 pi by 6 is equal to cos inverse of cos of 2 pi plus pi by 6 which is equal to cos inverse of cos of pi by 6 because cos of 2 pi plus theta is equal to cos theta which is equal to pi by 6 because pi by 6 belongs to 0, pi which is the principal value branch of cos inverse therefore the required answer is pi by 6 I hope you understood the problem. Bye and have a nice day.