HP 20S and HP 21S: 2 x 2 Matrices Inverse and Square
Let M be the 2
x 2 matrix:
M = [ [
R1, R2 ] [ R3, R4 ] ]
Store values in
R1, R2, R3, and R4 before executing the program.
HP 20S and HP 21S: Inverse of a Matrix
Output: Determinant of M in R0. If the determinant is 0, then the program terminates,
since the matrix is determined to be singular (and have normal inverse). Output:
M^1: [ [ R5, R6 ], [ R7, R8 ] ] where
R5 = R4/det(M)
R6 = R2/det(M)
R7 = R3/det(M)
R8 = R4/det(M)
The keystrokes
for the HP 20S and HP 21S are the same.
STEP

CODE

KEY

01

61, 41, A

LBL A

02

22, 1

RCL 1

03

55

*

04

22, 4

RCL 4

05

65



06

22, 2

RCL 2

07

55

*

08

22, 3

RCL 3

09

74

=

10

21, 0

STO 0

11

26

R/S

12

61, 43

X=0?

13

61, 26

RTN

14

22, 4

RCL 4

15

41, 1

XEQ 1

16

21, 5

STO 5

17

26

R/S

18

22, 2

RCL 2

19

41, 1

XEQ 1

20

32

+/

21

21, 6

STO 6

22

26

R/S

23

22, 3

RCL 3

24

41, 1

XEQ 1

25

32

+/

26

21, 7

STO 7

27

26

R/S

28

22, 1

RCL 1

29

41, 1

XEQ 1

30

21, 8

STO 8

31

61, 26

RTN

32

61, 41, 1

LBL 1

33

45

÷

34

22, 0

RCL 0

35

74

=

36

61, 26

RTN

Example 1:
[ [1.9, 7],
[3.5, 4.2] ]^1 = [ [ 0.2542, 0.4237], [ 0.2119, 0.1150 ]]
Determinant: 16.52
Example 2:
[ [2, 8],[5,
6] ]^1 = [ [0.1154, 0.1538], [ 0.0962, 0.0385] ]
Determinant: 52
HP 20S and HP 21S: Square of a Matrix
Output: [ [ M5, M6 ] [ M7, M8 ] ]
M5 = R1^2 + R2
* R3
M6 = R2 * (R1 +
R4)
M7 = R3 * (R1 +
R4)
M8 = R2 * R3 +
R4
STEP

CODE

KEY

01

61, 41, A

LBL A

02

22, 1

RCL 1

03

51, 11

x^2

04

75

+

05

22, 2

RCL 2

06

55

*

07

22, 3

RCL 3

08

74

=

09

21, 5

STO 5

10

26

R/S

11

22, 2

RCL 2

12

55

*

13

33

(

14

22, 1

RCL 1

15

75

+

16

22, 4

RCL 4

17

34

)

18

74

=

19

21, 6

STO 6

20

26

R/S

21

22, 3

RCL 3

22

55

*

23

33

(

24

22, 1

RCL 1

25

75

+

26

22, 4

RCL 4

27

34

)

28

74

=

29

21, 7

STO 7

30

26

R/S

31

22, 2

RCL 2

32

55

*

33

22, 3

RCL 3

34

75

+

35

22 ,4

RCL 4

36

51, 11

x^2

37

74

=

38

21, 8

STO 8

39

61, 26

RTN

Example 1:
[ [1.9, 7],
[3.5, 4.2] ]^2 = [ [ 28.11, 42.7], [ 21.35, 42,14 ]]
Example 2:
[ [2, 8],[5,
6] ]^2 = [ [ 44, 32 ], [ 20, 76 ] ]
Eddie
This blog is
property of Edward Shore, 2017
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