remember some triangle functions

(1)sinA+sinB=2sin((A+B)/2)cos((A-B)/2);

(2)sinA-sinB=2cos((A+B)/2)sin((A-B)/2);

(3)cosA+cosB=2cos((A+B)/2)cos((A-B)/2);

(4)cosA-cosB=-2sin((A+B)/2)sin((A-B)/2);

I haven't finish today's learning plan-only finished 4 pages of 10 pages.That's ok,I am a lazy man today.I am not a machine.