Evaluate the sum .
Note: The numbers are in degrees, example:
Let N be this sum.
If you replace the sinus with cosine, you'll get something interesting :
So replace for each term.
You'll have :
But we know that
So if we replace again :
The aim will be to retrieve N in the brackets. So tihere is missing and an extra .
sin(0)=0, so we can remove . As sin(90°)=1,
By using (1) :
And then you have N