The next step is :
cos(a/2) = 0 or cos (a) = 0 or cos (5a/2)=0
Solve each case using general solution of trigonometry, then find the particular solution for a
Prove that cos a + cos 2a + cos 3a + cos 4a = 4 cos (a/2)cos a cos (5a/2)
i can prove this .
hence , find all the values of a between 0 and 180 such that
cos a + cos 2a + cos 3a + cos 4a =0
so i know i should start like this
4 cos (a/2)cos a cos (5a/2)=0
but i am unsure how to continue here .