Himanshu
Last Activity: 9 Years ago
First of all, multiply 2 and then add 3 on both sides.
Resolve 3 as 1+1+1 on L.H.S and open the identities of cos(x-y).
Replace every 1 as sin2a + cos2a , sin2b + cos2b and sin2c + cos2c respectively.
You will get the two terms (cosa + cosb + cosc)2 + (sina +sinb + sinc)2 = 0
Hence, cosa + cosb + cosc = 0