program class5

      double precision x1,x2,t,dt,f1,f2,v1,v2,v3,v4,w1,w2,w3,w4

      open(unit=13,file="data.d")

      dt = 0.4
      x1 = 0
      x2 = 0

      do 10 t = 0, 12, dt
         write(13,*) t,x1,x2

         v1 = f1(x1,x2,t)*dt
         w1 = f2(x1,x2,t)*dt

         v2 = f1(x1+0.5*v1,x2+0.5*w1,t+0.5*dt)*dt
         w2 = f2(x1+0.5*v1,x2+0.5*w1,t+0.5*dt)*dt

         v3 = f1(x1+0.5*v2,x2+0.5*w2,t+0.5*dt)*dt
         w3 = f2(x1+0.5*v2,x2+0.5*w2,t+0.5*dt)*dt

         v4 = f1(x1+v3,x2+w3,t+dt)*dt
         w4 = f2(x1+v3,x2+w3,t+dt)*dt

         x1 = x1 + (v1+2*v2+2*v3+v4)/6.0
         x2 = x2 + (w1+2*w2+2*w3+w4)/6.0

 10   continue

      write(13,*) t,x1,x2

      stop
      end

      double precision function f1(x1,x2,t)
      double precision x1,x2,t

      f1 = x2

      return
      end

      double precision function f2(x1,x2,t)
      double precision x1,x2,t

      f2 = 3*sin(2*t)-4*x1-0.01*x2

      return
      end