Question 725643
simplify the ff: 
cos(p+z)-cos(p-z)
Identity: cos s-cos t=-2 sin (s+t)/2 sin (s-t)/2
cos(p+z)-cos(p-z)
=-2 sin (p+z+p-z)/2 sin (p+z-p+z)/2
=-2 sin (2p)/2 sin (2z)/2
=-2 sin(2p)sin(2z)/4
=-sin(2p)sin(2z)/2