the wave equation is f(x,t) = A2 - Acos(2wt - 2kx)2
wave welocity = w/k
for calculating particle velocity, let us fix a particular position (x=0 for convenience)
then f(t) = A2 - Acos(2wt)2
f'(t) = wAsin(2wt)
so, vmax (of particle) = wA = w/k
=> A = 1/k