Two simple harmonic waves having equal amplitudes of 8cm and equal...

If two waves have the same amplitude, frequency, and direction, but are out of phase by $ϕ$ degrees, then the amplitude of the resultant waves is given by ${A}_{R} = 2Acos\left(\dfrac{\Deltaϕ}{2}\right)$

Here, ${A}_{1} = {A}_{2} = {A}_{R} = 8cm$

So, $\cos \left(\dfrac{\Deltaϕ}{2}\right) = \dfrac{1}{2}$

Or phase difference between the individual waves is $\dfrac{\Deltaϕ}{2} = 60 ° ⇒ \Deltaϕ = 120 °$.