Вычислите 1-2 cos^2*13/cos 26

Question

Вычислите 1-2 cos^2*13/cos 26

Answer 1

(1-2*(cos13)²)/cos26.  ((sin13)²+cos(13)²-2*(cos13)²)/cos26=
=((sin13)²-(cos13)²)/cos26= -((cos13)²-(sin13)²)/cos26=-cos26/cos26=-1.