algebraic sines and cosines
For any rational number , the sine and the cosine of the number are algebraic numbers
.
Proof. According to the http://planetmath.org/node/11664parent entry, and can be expressed as polynomials with integer coefficients of or , respectively, when is an integer. Thus we can write
where . If where are integers and , we have
i.e. both and satisfy an algebraic equation. Q.E.D.
For example,
whence we have the identity
and therefore is algebraic over .