8900  DEF FNF()=-X/
9000  REM *****  DEZ10R  ***** MATHEMATICS PROGRAM
9001  REM  *****  VERSION 1  *****  7/31/69  *****
9002  REM  SOLVES A FIRST ORDER DIFFERENTIAL EQUATION
9003  REM DEF FNF(Y) = ??
9004  READ X0,Y,H,L,B
9005  LET X=X0
9006  LET E=1/L
9007  IF (B-X0)*H>0 THEN 9009
9008  LET H=-H
9009  IF (B-X0)*L>0 THEN 9011
9010  LET L=-L
9011  PRINT "VALUE OF X","VALUE OF Y"
9012  PRINT 
9013  PRINT X,Y
9014  GOTO 9018
9015  LET P=Y+H*FNF(Y)
9016  LET X=X+H
9017  LET Y=(Y+P)/2+H/2*FNF(P)
9018  IF (X+H-B)*SGN(L)+1.E-07 >= 0 THEN 9021
9019  LET R=X+H
9020  GOTO 9022
9021  LET R=B
9022  LET A=INT(E*R)/E+L*(SGN(H)-1)/2
9023  IF (A+L-R)*SGN(L)-1.E-07 >= 0 THEN 9025
9024  LET A=A+L
9025  IF (X-A)*SGN(L)+1.E-07 >= 0 THEN 9028
9026  LET Q=A
9027  GOSUB 9034
9028  IF R=B THEN 9030
9029  GOTO 9015
9030  IF ABS(R-A)<.5*.000001 THEN 9033
9031  LET Q=R
9032  GOSUB 9034
9033  STOP 
9034  LET P1=Y+(Q-X)*FNF(Y)
9035  LET X1=X
9036  LET X=Q
9037  LET Y1=(Y+P1)/2+(Q-X1)/2*FNF(P1)
9038  LET X=X1
9039  PRINT Q,Y1
9040  RETURN 
9900  DATA 0,1,.01,.1,1
9999  END 
