On the first, you have formulas for cos(a+-b). Use them on the right, and it will simplifiy quickly.
On the second, I wouldstart with the double angle formulas...on the left.
Prove that
2sinxsiny = cos(x-y) - cos(x+y)
Prove that
sin2x + sin2y = 2sin(x+y)cos(x-y)
1 answer