The Museum of HP Calculators

Calculus And Roots Of f(X) for the HP-67

This program is Copyright © 1976 by Hewlett-Packard and is used here by permission. This program was originally published in the HP-67 Standard Pac.

This program is supplied without representation or warranty of any kind. Hewlett-Packard Company and The Museum of HP Calculators therefore assume no responsibility and shall have no liability, consequential or otherwise, of any kind arising from the use of this program material or any part thereof.

Card Labels

Calculus And Roots Of f(X)









x->f '(x)











This program incorporates four routines for numerical analysis of user specified functions. Suppose the figure below represents a known function of x called f(x).

If the formula for f(x) can be keyed into program memory in less than 112 steps (including LBL and RTN), this program can be used to find the value of f(x) at any point x, the derivative of f(x) at any point x, the integral of f(x) over a specified interval and the real roots of f(x). There may be up to five different f(x) functions in program memory at one time. They must be labeled from 1 to 5. The function to be evaluated is selected by keying in 1, 2, 3, 4 or 5 and pressing A.

Only side 1 of Calculus and Roots of f(x) is used for the program. In the listing below, side two (steps 112-223) lists functions used in the examples. You may wish to add functions you use frequently to side 2 of this card or record them on additional magnetic cards. Once recorded, the functions can be linked to Calculus and Routs of f(x) by the following sequence of operations:

  1. Load side 1 of Calculus and Roots of f(x).
  2. Press GTO . 112
  3. Press g MERGE.
  4. Load your magnetic card.

Once a function is defined and selected, keying in a value of x and pressing the C key will result in the evaluation of f(x) (see below).

Similarly, the value of the slope of f(x) at a particular point x can be calculated by keying in x and pressing the B key (see figure below). The slope of f(x) is determined using an approximation to the differential:

The value of deltax used to approximate the differential is assumed to be 0.01% of x (10-4 * x) unless a %%delta is specified by the user. That is:

deltax = %delta/100 * x

In the special case where x = 0, deltax is set equal to %delta.

For most applications, the assumed value of 0.01% should be adequate. In some cases more accurate results can be obtained using a smaller value of %delta. However, care must be taken to assure that the calculator can accurately resolve the difference between f(x- deltax/2) and f(x + deltax/2).

The D key may be used to approximate the integral or area under a curve.

You specify the end points of the interval (a and b) and the number of rectangles (n) the interval should be broken into (see figure above). The calculator computes the sum of the areas of the rectangles. The more rectangles used the closer this value is to the actual area under the curve. However, more rectangles mean more computation time. Experience with a particular function should lead to a balance between accuracy and execution time.

Root finders are used to solve equations which are difficult or impossible to solve explicitly. An example of such an equation is

     f(x) = ln x + 3x - 10.8074 = 0
which is solved in example 4.

The root finder incorporated in this program uses a secant method of approximation. You must supply the routine with an initial guess of the root. Based on this guess, it will attempt to make better and better approximations of the root by the following formula:

xi+1 = xi - f(xi)[(xi - xi-1)/(f(xi) - f(xi-1))]

The display is automatically set to fix mode during the root finder portion of the program. When the last approximation is accurate to the number of places specified by the display setting of the calculator, the routine halts and displays the root.

Since the root finder starts its search based on your guess, care should be exercised in guess selection. A bad guess will cause long execution times and could result in a machine status error halt (overflow, division by zero, log of a negative number, etc.). If this happens, simply try another guess. Practice will make the pitfalls more obvious and easier to avoid.

A special feature of the iterative routine is the pause function. This feature allows the program to pause at one point in each iteration to display the current approximation of the root. The pause option may be turned off and on by pressing f E. The pause allows you to watch the routine converge (or diverge) without interrupting the program. This can be a helpful tool when the iterative routine fails to converge. By watching each successive approximation of the root, the reasons for failure of convergence can usually be determined.


The value of x is stored in R0 by the program. It is also in the X register when control transfers to the function subroutine.

Registers R1-R8, and RS0-RS9 are available for use in f(x) or for other user storage.

User-specified functions may use one level of subroutine nesting.

The secant method does not guarantee convergence to a root. If it fails, use Solution to f(x) on an interval.

Given one guess, the root finder will find, at most, one root of an equation. Other real roots, if they exist, may be found by modifying the initial guess.

In order to compute f'(x), the function f(x) must be continuous on the interval (x + deltax/2, x - deltax/2).




Input Data/Units


Output Data/Units


Load side 1.



Load subroutine(s) (either key them in or link from program step 112).





Select function label number.

i (1-5)




Store any constants necessary to subroutine(s) loaded in step 2.





For differentiation, go to step 6. For evaluation of a function, go to step 9. For integration of a function, go to step 11. To find a root, go to step 15.





Optional: Key in percent delta.


f A



Key in x and calculate derivative at x.





For new x, go to step 7. For a new case, go to step 2, 3, 4, 5 or 6.





Key in x and evaluate function.





For new x, go to step 9. For a new case, go to step 2, 3, 4, or 5.





Input the number of intervals.





Input the lower limit.





Input the upper limit and calculate the integral.





For new limits or interval, go to step 11. For a new case, go to step 2, 3, 4 or 5.





Optional: Key in percent delta.


f A



Optional: Toggle pause mode.


f E

1.00 or 0.00


Key in guess and calculate root.





For a new guess go to step 17. For a new case go to step 2, 3, 4 or 5.




Example 1

Numerical integration provides the only solution to the complete elliptic integral of the first kind:

     u = intrgtl(0..pi/2)dtheta/sqrt(1-K2sintheta2)

Find the value of u for limits of integration of 0.0 to pi/2 Let K be 0.5 and store it in register 1 for access by the program. Use 3 and then 10 for the number of intervals. The formula for the integral is listed under label three of the listing below. If either example 2 or example 3 has just been run, skip the first three lines under keystrokes.

Keystrokes                          Outputs
Load side 1 only
Load side 2 (If you've entered 
             the examples)
Select label 3                        3.00
3 A
0.50 STO 1                            0.50
Integrate using 3 intervals
h pi 2 ÷ D                     1.685750251
Integrate using 10 intervals
h pi 2 ÷ D                     1.685750355

Example 2

In the design of gear teeth, it is frequently necessary to calculate x for a given value of the involute:

      INV(x) = tan x - x

    or restated

      f(x) = tan x - x - INV(x) = 0 

If the involute of x is 0.0049819, what is x? This problem requires an iterative solution since the equation cannot be explicitly solved for x. Use 0.21 radians as your initial guess. The equation for f(x) is listed under label 2 in the program listing. Use the pause feature to watch the routine converge. Skip the first three lines under keystrokes if Example 1 or 3 has been run. Store the involute (.0049819) in R2 for access by the function.

Keystrokes                      Outputs
Load side 1 only
GTO . 112 MERGE 
Load side 2 
Select label 2 
2A                                2.00
Set pause
DSP 2 f E                         1.00
.0049819 STO 2 .21 E              "0.25"
                                  0.24     (rad)

Example 3

In many instances, a function is represented graphically. This program can be of use in integration and, in some cases, differentiation of such graphs. Label 1 in the listing below is designed for this purpose. It returns x values to the display. You must find f(x) from the graph, key it in and press R/S. For the function below find the integral from a to b using 5 intervals. Then find the derivative at a, using 10% for %delta. After the problem is complete, return % delta to 0.01%. If either Example 1 or Example 2 was run previously, skip the first three lines under keystrokes.

Keystrokes                     Outputs
Load side 1 only
GTO . 112 g MERGE
Load side 2
Select Label 1
1 A                             1.00
Key in integration limits 
and return first x value
5 ENTER 1.40 ENTER 4.70 D       1.73     (x)
From the graph, f(x) at 
x = 1.73 equals 14.2.
Key 14.2 in and press R/S. 
The next value
of x will be displayed.
14.2 R/S                        2.39
f(2.39) = 16
16 R/S                          3.05
f(3.05) = 17
17 R/S                          3.71
f(3.71) = 16.9
16.9 R/S                        4.37
f(4.37) = 15.3
15.3 R/S                       52.40     (answer)
To find the derivative 
at point a
10 f A 1.40 B                   1.33     (x - deltax/2)
f(1.33) = 12.7
12.7 R/S                        1.47     (x + deltax/2)
f(1.47) = 13.3
13.3 R/S                        4.29     (slope)
Return %deltato 0.01%
.01 f A                         0.01

Example 4

Find the root of ln x + 3x - 10.8074 = 0. Determine the slope at the root. This equation is not listed below. It should be keyed into program memory starting at step 112. Use R1 to store the 3 and R2 to store 10.8074.

Keystrokes                     Outputs
Load side 1 only
GTO . 112
Switch to W/PRGM               112 35 22
f LBL 1                         31 25 01
f LN                           114 31 52     (ln x)   
RCL 1                          115 34 01                  
RCL 0                          116 34 00                  
x                              117    71                  
+                              118    61     (ln x + 3x) 
RCL 2                          119 34 02                  
-                              120    51     (ln x + 3x -
h RTN                          121 35 22                 
Switch to Run
Select LBL 1
1 A                            1.00         
3 STO 1                        3.00         
10.8074 STO 2                  10.81            
Make a guess of 5.0                         
5 E                            3.21 (ROOT)  
Find the derivative                         
B                              3.31 f'(3.21)

The Program

001  *LBL A     Store function number.
002   STO I
003   RTN
004  *LBL e     Pause toggle.
005   F0?
006   GTO 0
007   SF 0
008   1
009   RTN
010  *LBL 0
011   0
012   CF 0
013   RTN
014  *LBL a     Store %delta and set flag.
015   SF 1
016   STO E
017   RTN
018  *LBL B     Choose default %delta or use 0.01%?
019   EEX
020   CHS
021   2
022   RCL E
023   F1?
024   X<>Y
025   roll dn
026   %         If x=0 use %delta rather than % of x as deltax.
027   X=0?
028   LST X
029   STO C
030   2
031   ÷         f(x-deltax/2)
032   -
033   STO A
034   STO 0
035   GSB (i)
036   STO D
037   RCL A     [f(x+deltax/2)-f(x-delta/2)]/deltax
038   RCL C
039   +
040   STO 0
041   GSB (i)
042   STO B
043   RCL D
044   -
045   RCL C
046   ÷
047   RTN
048  *LBL C     f(x).
049   STO 0
050   GSB (i)
051   RTN
052  *LBL D     Store a.
053   X<>Y
054   STO 0     b-a.
055   -
056   X<>Y      Store n.
057   STO B
058   ÷
059   STO C     (b-a)/n
060   2
061   ÷         (b-a)/2*n
062   STO + 0
063   0         Set integral sum at 0.
064   STO 9
065   RCL B     Put number of intervals in I.
066   X<>I
067  *LBL 7
068   X<>I      Return number of intervals in I.
069   STO B
070   RCL 0
071   GSB (i)   F'(R0)
072   RCL C
073   STO + 0   R0 + (b-a)/n
074   x
075   STO + 9
076   RCL B     Decrement n and save function in display.
077   X<>I
078   DSZ I 
079   GTO 7     Store function number.
080   STO I
081   RCL 9     Display result of integration.
082   RTN
083  *LBL E     Use numerical differentiation to generate xi from use guess.
084   FIX
085   GSB B
086   RCL B
087   GTO 0
088  *LBL 6     Evaluate f(xi)
089   RCL 0
090   GSB (i)
091   STO B
092  *LBL 0
093   RCL A     Secant method calculates correction for x value
094   RCL 0     and sets values for next loop.
095   STO A
096   -
097   RCL D
098   RCL B
099   STO D
100   -
101   ÷
102   x
103   STO - 0   Subtract Correction.
104   RCL 0
105   F0?       Pause and display root if flag set?
106   PAUSE
107   ÷         RND(change/xi+1)
108   RND
109   X!=0?     Accurate to display?
110   GTO 6
111   RCL 0     If it is, display result.
112   RTN

001  *LBL 1     Graphical evaluation subroutine.
002   R/S
003   RTN
004  *LBL 2
005   RAD
006   TAN       f(x) = tan(x) - Inv(x) - x
007   LST X
008   -
009   RCL 2
010   -
011   DEG
012   RTN
013  *LBL 3
014   RAD
015   SIN
016   RCL 1     f(theta) = 1/sqrt(1-k2sin2theta)
017   x
018   X2
019   1
020   X<>Y
021   -
022   sqrt
023   1/X
024   DEG
025   RTN

Register Use

R0  x        
R9  integral
A   xi-1
B   f(xi)
C   deltax
D   f(xi-1)
E   deltax
I   function

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