(6.) Different arcs whose sine, or cosine is the same.

=

sin (2n+1.π-a) = − sin (2n + 1.π+a).

cos (2nπ + α)
cos (2n+1.π-a) =

cos (2nπ-a),

cos (2n+1.π + α),

cos b=sin (a+b). sin a + cos (a + b). cos a.
sin (n + 2) a + sin na = 2 sin (n + 1) a. cos a.
sin (n + 2) a-sin na=2 cos (n + 1) a. sin a.
cos na + cos (n + 2) a = 2 cos (n + 1) a. cos a.
cos (n + 2) a = 2 sin (n + 1) a. sin a.
sin (n+2) a. sin na = sin (n + 1) a ]2 — (sin a)2.

cos na

2

(12.) Values of the sine, cosine, &c. of 30°, 45°, and 60°.