a=Asinωt0...(1)
b=Asin2ωt0...(2)
c=Asin3ωt0...(3)
adding equation (1) and (3)
a+c=A[sinωt0+sin3ωt0]=2Asin2ωt0cosωt0...(4)
∵sinC+sinD=2sin2C+Dcos2C−D
from equation (2) and (4)
ba+c=2cosωt0
ω=t01cos−1(2ba+c)⇒f=2πt01cos−1(2ba+c)