I was reading through a text book on trigometric identities and gave the definition:

- cos(a) = sin(a + 90) // (1a)
- sin(a) = cos(a - 90) // (2a)

then it went to derive another two new identities from the above identities

- cos(a) = -sin(a-90) // (1b)
- sin(a) = -cos(a+90) // (2b)

What I don't understand is steps involved from deriving (1a) to (1b) and (2a) to (2b), any help would be appreciated