In this case or scenario,
the double-angle identity that should be used is the one for cosine.
In totality, we shall need the following three trigonometric
identities to end up with the equality:
1. cos (2a) = cos² (a) - sin² (a)
2. sin² (a) + cos² (a) = 1
3. tan² (a) + 1 = sec²
(a)
Using identities 1 and 2 on the left-hand side of the
equation, we get the following:
1 + cos (2a) = 1 + cos² (a) - sin² (a) = 2 cos² (a)
Recalling that cos² (a) = 1 / sec² (a) and applying identity
3, we find the following:
2 cos² (a) = 2 / sec² (a) = 2 / (1 + tan² (a))
Therefore giving us:
2 cos² (a) = 2 / (1 +
tan² (a))
