Cos (pi/11) + cos (3pi/11) + cos (5pi/11) + cos (7pi/11) + cos (9pi/11)

All you have to do is input these terms in a scientific calculator and you would immediately get the answer. However, you should note that the units are in radians. Therefore, make sure that your calculator is in the mode of radians before inputing. Then, you would get an answer of 1/2.