1)sin3b+sin5b=2sin3b+5b/2*cos3b-5b/2= 2sin4b*cos(-b)=
=2sin4b*cosb
2)sin15-sin25=2sin(15-25/2)*cos(15+25/2)= 2sin(-5)cos20=-2sin5cos20
3)sin140-sin20=2sin140-20/2*cos140+20/2=2sin60*cos80=2*√3/2cos80=
= √3cos80
4)cos13a-cos5a=-2sin13a+5a/2*sin13a-5a/2=-2sin9a*sin4a
5) cos78+cos18=2cos78+18/2*cos78-18/2=2cos48*cos30=2*√3/2cos48=
= √3*cos48