summation of sine angle in AP
All would have been well, if you correctly used $$1+\cos 2x=2\cos^2x$$
(There are lots of cancelling terms. Then the only thing you have to remember to get the answer is $\cos\frac{\pi}{5}=\frac{\sqrt5+1}4.$)
All would have been well, if you correctly used $$1+\cos 2x=2\cos^2x$$
(There are lots of cancelling terms. Then the only thing you have to remember to get the answer is $\cos\frac{\pi}{5}=\frac{\sqrt5+1}4.$)