How to find cos2θ if sinθ = 2?
How to find cosθ if sin2θ = 3?
Both lie in quadrant I. I'm unsure how to convert from one identity to the other ...
In this post ...
Double Angle Equations!
...you were requested to post specific problems rather than cryptic references to double angle problems. Now it seems you're making up stuff, because you really don't know what you're doing.
Post a specific problem from a text or as given by your instructor.
But please let us know what the actual problem statements are. You have been asked to clarify a question before.