400 "V" CLEAR : CLS : INPUT "x=";X#,"n=";K: DIM A#(K):E=1E-8:H=.2: GOSUB 405:A#(0)=0: FOR J=2 TO K
  401 H=H/2: GOSUB 405:A#(J)=G#:L=1: FOR I=J-1 TO 1 STEP -1:L=L*4
  402 A#(I)=(L*A#(I+1)-A#(I))/(L-1): NEXT I: IF J=2 THEN 404
  403 IF  ABS (A#(1)-D#)<E* ABS A#(1) PRINT "f"+ CHR$ 96+"(x)=";A#(1): END 
  404 D#=A#(1): NEXT J: PRINT "too slow": END 
  405 A#=X#:X#=X#+H: GOSUB 1:B#=F#:X#=A#:X#=X#-H
  406 GOSUB 1:C#=F#:X#=A#:G#=(B#-C#)/(2*H): RETURN