Formulas
Derivation
(i)
First, we can apply trigonometric addition formulas.
By the symmetry of the cosine function about the x-axis, we can derive the result as follows.
(ii)
Derivation of (ii) is similar to the one in (i).
(iii)
Applying trignometric addtion formulas is same.
However, by the symmetry of the sine function about the origin, the result of the formula is always 0.