HP 71B Programs: September 2025
One of my favorite calculators/pocket calculators of all time is the HP 71B.
ADDMOD: (a + b) mod n = a mod n + b mod n
10 DESTROY A, B, N
20 DISP “(A+B) MOD N” @ WAIT .5
30 INPUT “A? “; A
40 INPUT “B? “; B
50 INPUT “C? “; C
60 S = MOD(A, N) + MOD(B, N)
70 S = MOD(S, N)
80 DISP “SUM = “; S
MULTMOD: (a * b) mod n = a mod n * b mod n
10 DESTROY A, B, N
20 DISP “(A*B) MOD N” @ WAIT .5
30 INPUT “A? “; A
40 INPUT “B? “; B
50 INPUT “N? “; N
60 P = MOD(A, N) * MOD(B, N)
70 P = MOD(P, N)
80 DISP “PRODUCT = “; P
Examples:
A |
630 |
48 |
15 |
B |
320 |
99 |
47 |
N |
700 |
15 |
7 |
SUM: |
250 |
12 |
6 |
PRODUCT: |
0 |
12 |
5 |
GABLE: Area of a Gable Roof
10 DESTROY P, S, L
20 DEGREES
30 INPUT “PITCH (IN.)? “; P
40 INPUT “SPAN (FT.)? “; S
50 INPUT “LENGTH (FT.)? “; L
60 A = 2 * S * L / COS(ATAN(P/12))
70 DISP “AREA = “; A; “ FT^2”
Input: Pitch (P) |
6” |
3” |
4” |
Input: Span (S) |
110” (110/12 ft) |
5’ 8” (5 + 8/12 ft) |
15 ft |
Input: Length (L) |
100” (100/12 ft) |
10’ |
10 ft |
Output: Area (A) |
170.81074828 ft^2 |
116.821326059 ft^2 |
316.227766017 ft^2 |
INTENSE: Light Intensity of a Spherical or a Cylindrical Light Source
intensity = power / surface area
intensity (W/m^2), power (W), surface area (m^2)
Surface area: sphere = 4 * π * r^2, cylinder = 2 * π * (r * h + r^2)
10 DESTROY P, K$, R, H
20 DISP “INTENSITY OF LIGHT!” @ WAIT .5
30 INPUT “POWER (W)? “; P
40 DISP “1. SPHERE 2. CYLINDER”
50 DELAY 0,0
60 K$ = KEY$
70 IF K$ = “1” OR K$ = “S” THEN GOTO 100
80 IF K$ = “2” OR K$ = “C” THEN GOTO 200
90 GOTO 40
100 INPUT “RADIUS (M)? “; R
110 A = 4 * PI * R^2
120 GOTO 300
200 INPUT “RADIUS (M)? “; R
210 INPUT “HEIGHT (M)? “; H
220 A = 2 * PI * (R * H + R^2)
230 GOTO 300
300 I = P / A
310 DISP I; “W/M^2”
Examples:
Sphere: r = 4 m, Power = 60 W: Intensity = .298415518297 W/M^2
Cylinder: r = 1.25 m, h = 0.75 m, Power = 70 W; Intensity = 4.45633840656 W/M^2
SIMPLE: Simple Interest – Banker’s Rule
interest = amount * rate% / 100 * #days / 360
final = principal + interest (final, maturity value)
10 DESTROY P, R, I, M
20 DISP “SIMPLE INTEREST” @ WAIT .5
30 INPUT “AMT? “; P
40 INPUT “RATE%? “; R
50 INPUT “# DAYS? “; D
60 I = P * R * D / 360 / 100
70 M = P + I
80 IMAGE K, M10D.2D
90 DISP USING 80; “INT.=”, I @ PAUSE
100 DISP USING 80; “FINAL=”, M
Notes:
* When the HP 71B is in pause, press [ f ] [ + ] to continue (CONT).
* To stop execution, press [ ON ] (ATTN).
* IMAGE K, M10D.2D: display “prompt string”, number rounded to 2 decimal places. The number is right-justified, make the prompt string 7 characters or less to fit both the string and number on the screen.
Examples
Input: Amount |
1500 |
2000 |
2800 |
Input: Rate % |
4.00% |
8.00% |
6.65% |
Input: # Days |
180 |
90 |
60 |
Output: Interest |
30 |
40 |
31.03 |
Output: Final |
1530 |
2040 |
2831.03 |
AIRSOUND: Speed of Sound in Air
S = 331.5 + .606 * T
S = speed of sound in air, m/s
T = temperature in °C
10 DESTROY K$, S, T
20 DISP “SPEED OF SOUND IN AIR” @ WAIT .5
30 DISP “SOLVE FOR…” @ WAIT .5
40 DISP “1. SPEED 2. TEMP”
60 K$ = KEY$
70 IF K$=”1” OR K$=”S” THEN GOTO 100
80 IF K$=”2” OR K$=”T” THEN GOTO 200
90 GOTO 40
100 INPUT “TEMP (°C)? “; T
110 S = 331.5 + .606 * T
120 DISP “S=”; S; “ M/S”
130 END
200 INPUT “SPEED (M/S)? “; S
210 T = (S- 331.5) / .606
220 DISP “T=”; T ; “°C”
230 END
Examples:
Input: T = 74 °F = (32 + 1/3) °C Output: S = 345.64 m/s |
Input: S = 350 m/s Output: T = 30.5280528053 °C |
Input: T = -8.5 °C Output: S = 326.349 m/s |
Input: S = 382 m/s Output: T = 83.3333333333 °C |
Eddie
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