Let the amplitudes of the individual motion be A each. The resultant ampliotude is also A. If the phase difference between the two motion is `sigma`
`A = sqrt(A^(2) + A^(2) + 2 A A cos sigma)`
or `= A sqrt(2 (1 + cos sigma) = 2 A cos ((sigma)/(2))`
or `cos ((sigma)/(2)) = (1)/(2)`
or `sigma = 2 pi//3`