20.2 The Wave Equation We take the 2nd derivative of a- goei(w(t-2/v) w.r.t. the time t We take the 2nd derivative of a= coei(w(t-2/u) w.r.t. the coordinate 2. 02a(2) From these two equations we get 02a(2m)2a2a1a2 a22a2入2at2 O This is the partial differential equation of waves in 1-dim case