61. Table of Useful Formulæ. -The following is a list of important formulæ proved in this chapter, and summed for the convenience of the student: up 7. 2 cos x cos y = cos x cos y + sin x siny. = sin (x + y) + sin (x − y) = sin (x + y) sin (x − y). = cos (x + y) + cos (x − y). 8. 2 sinx siny = cos(x − y) — cos (x + y). 9. sinx + sin y = 2sin(x + y) cos † (x − y). 10. sinx siny 2 cos(x+y) sin(x − y). = |