10 CLS
20 PRINT "DERIVATIVE"
30 DEFDBL A-Z 
40 DEFSNG N
50 PRINT 
60 REM - ENTER FUNCTION AT LINE 200
70 PRINT "(ENTER X=99999 TO END PROGRAM)" 
80 PRINT "DERIVATIVE AT X=";
90 INPUT X1
100 IF X1=99999 THEN 220  
110 D=0 
120 FOR N=1 TO 10 
130 D1=D
140 X=X1+.5^N 
150 Z=X : GOSUB 200  : F=Z : Z=X1 : GOSUB 200  
160 D=(F-Z)/(X-X1)
170 NEXT N 
180 PRINT "                    IS";2*D-D1 
190 GOTO 80   
200 REM - ENTER FUNCTION HERE (Z="FUNCTION")
210 RETURN
220 END