The equations of motion of two waves propagating in the same direction in a medium is given by
y1=A sin(ωt−kx)
and y2=A sin(ωt−kx−θ).
Then amplitude of the particle in this medium is
Resultant amplitude when waves of phase difference θ, interfere is given by
y0=√A21+A22+2A1A2(cosθ)
HereA1=A2=A
⇒y0=A√2(1+cosθ)=2A cos θ2