Given that, amplitude and frequency of two waves are same. Hence, resultant amplitude due to superposition of two waves with phase difference φ is given by A2‌=A12+A22+2A1A2‌cos‌φ ⇒A2‌=A2+A2+2A2‌cos‌φ‌‌(∵A1=A2=A) ⇒A2‌=2A2+2A2‌cos‌φ ⇒cos‌φ=‌