Question: why are cosine and secant even functions?
Answer: We know this is true because of the negative angle identities for cosine and secant. ... Specifically, the periods of sine, cosecant, cosine, and secant are 2π, and the periods of tangent and cotangent are π. To understand why, move θ from 0 all the way to 2π on the unit circle and check how each function changes.