Three waves of equal frequency having amplitudes  arrive at a given point with successive phase difference of , the amplitude of the resulting wave in  is given by

The amplitudes of the waves are

 and

and the phase difference between 1^st and 2^nd wave is  and that between 2^nd and 3^rd is . Therefore, phase difference between 1^st and 3^rd is . Combining 1^st with 3^rd, their resultant amplitude is given by

Or

 in the direction of first

Now combining this with 2^nd wave we have, the resultant amplitude

Or