Exercise 3.78.  

Figure 3.35:  Signal-flow diagram for the solution to a second-order 
linear differential equation.

Consider the problem of designing a signal-processing system to study 
the homogeneous second-order linear differential equation

The output stream, modeling y, is generated by a network that contains a 
loop. This is because the value of d2y/dt2 depends upon the values of y 
and dy/dt and both of these are determined by integrating d2y/dt2. The 
diagram we would like to encode is shown in figure 3.35. Write a 
procedure solve-2nd that takes as arguments the constants a, b, and dt 
and the initial values y0 and dy0 for y and dy/dt and generates the 
stream of successive values of y.

————————————————————————————————————————————————————————————————————————

(define (solve-2nd a b dt y0 dy0)
  (define y (integral (delay dy) y0 dt))
  (define dy (integral (delay ddy) dy0 dt))
  (define ddy (add-streams (scale-stream dy a)
                           (scale-stream y b)))
  y)