 Hello and welcome to the session. Let us understand the following problem. Cos inverse of cos of 7 pi by 6 is equal to a 7 pi by 6, b 5 pi by 6, c pi by 3, d pi by 6. Now let us write the solution. We know that Reign law principle value branch 6 is 0 to pi and 7 pi by 6 does not belong to 0 to pi. Therefore cos inverse of cos of 7 pi by 6 is equal to cos inverse of cos of pi plus pi by 6 which is equal to cos inverse of minus cos of pi by 6 which is equal to cos inverse of cos of pi minus pi by 6 because cos of pi minus theta is equal to minus cos theta which is equal to cos inverse of cos of 5 pi by 6 which is equal to 5 pi by 6 because 5 pi by 6 belongs to 0 to pi. Therefore the correct answer is b. I hope you understood the problem. Bye and have a nice day.