## Saturday, September 15, 2018

### HP 20S and 21S: Heron's Triangle

HP 20S and 21S:  Heron's Triangle

Introduction

The following program calculates the area of a triangle knowing the three lengths.  The area is determined by Heron's formula:

Area:  √(s * (s - a) * (s - b) * (s - c)) where s = (a + b + c)/2
Store the lengths of the triangle in registers R0, R1, and R2 respectively.
Results are stored in the following registers:
R3:  s
R4:  area

HP 20S and 21S Program: Heron's Triangle

The key codes for both calculators are the same.

STEP KEY KEY CODE
01 LBL A 61, 41, A
02 RCL 0 22, 0
03 STO 3 21, 3
04 RCL 1 22, 1
05 STO+3 21, 75, 3
06 RCL 2 22, 2
07 STO+3 21, 75, 3
08 2 2
09 STO÷3 21, 45, 3
10 RCL 3 22, 3
11 STO 4 21, 4
12 RCL 3 22, 3
13 - 65
14 RCL 0 22, 0
15 = 74
16 STO×4 21, 55, 4
17 RCL 3 22, 3
18 - 65
19 RCL 1 22, 1
20 = 74
21 STO×4 21, 55, 4
22 RCL 3 22, 3
23 - 65
24 RCL 2 22, 2
25 = 74
26 STO×4 21, 55, 4
27 RCL 4 22, 4
28 11
29 STO 4 21, 4
30 RTN 61, 26

Example

R0 = 17, R1 = 18, R2 = 21.   Result:  Area:  148.833238744

Note:  I switched computers last week.  For the last few years, I used Microsoft Word but for the time being I am going to use WordPad.  Let's see how this goes.  Fortunately I can still can use Unicode characters with Alt+X.  For example, I can type 221A, then type [Alt] + [ X ] to get the square root character.  Unfortunately, WordPad doesn't have spell check.

Eddie

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