Given that, amplitudes of two waves are a1=a2=10‌mm Phase difference, ϕ=90° Let A be the resultant amplitude, then by using expression, A=√a12+a22+2a1a2‌cos‌ϕ Substituting the values, we get A=√(10)2+(10)2+210)(10)×cos‌90° =10√2‌mm