For question 11 (which I think is more straightforward than 10), I think I have the answer, which is 2decimal places

For question 10, I end up with the following formula

R9 = (Cos(w) (2-pi/2)^{9})/9!

However, I have never understood what w is (some textbooks and videos use z) other than it is a number between x and c. I assume x is 2 and c is pi/2 but what number is inbetween. Depending on what I use I get a massive difference in "error". What is w?