More elementarily: cos x and sin x are the x and y coordinates of the point on the unit circle at an angle x anticlockwise from the positive x-axis. (I think this is the correct version of the “triangles” definitions.)
More elementarily: cos x and sin x are the x and y coordinates of the point on the unit circle at an angle x anticlockwise from the positive x-axis. (I think this is the correct version of the “triangles” definitions.)