nomor 2 mohon bantuannya

a. cos 110+cos 10+cos 130=0
2 cos 1/2 (110+10) cos 1/2 (110-10)+cos130=0
2 cos 1/2 (120) cos 1/2 (100)+cos 130=0
2 cos 60 cos 50+cos 130=0
2(1/2) cos 50+cos 130=0
cos 50 + cos 130=0
cos 130 + cos 50=0
2 cos 1/2 (130+50) cos 1/2 (130-50)=0
2 cos 1/2 (180) cos 1/2 (80) =0
2 cos 90 cos 40 = 0
2(0)(cos 40) =0
0(cos 40) = 0
0 = 0

b. cos 140+cos 20+cos 100=0
2 cos 1/2 (160) cos 1/2 (120) +cos 100=0
2 cos (80) cos (60)+cos 100=0
2 cos 80 (1/2)+ cos 100=0
cos 80 + cos 100=0
cos 100 + cos 80=0
2 cos 1/2 (180) cos 1/2 (20) = 0
2 cos (90) cos (10) = 0
2 (0) (cos 10) = 0
0=0