solve 3x3 system on 33s Message #1 Posted by martin cohen on 30 May 2004, 11:57 a.m.
Here is some code for the 33s (originally written for the 32sii) for solving a 3x3 linear system of equations.
The code uses Cramer's rule ( with determinants) and does not change the coefficients.
Solve a 3 x 3 system of linear equations
A*X + B*Y + C*Z = D
E*X + F*Y + G*Z = H
I*X + J*Y + K*Z = L
DET =
A B C
E F G
I J K
=
A * (F K  G J)  E * (B K  C J) + I * (B G  C F)
D_X =
D B C
H F G
L J K
Same for D_Y and D_Z
x = D_X/DET,, ...
===============================================================================
S: do via determinants and column swapping
(20 steps, 43.5, E8A7):
LBL S
XEQ U; STO W; VIEW W
1; XEQ V; XEQ U; RCL/ W; STO X; 1; XEQ V
2; XEQ V; XEQ U; RCL/ W; STO Y; 2; XEQ V
3; XEQ V; XEQ U; RCL/ W; STO Z; 3; XEQ V
RTN
U: determinant of [ [a, b, c], [e, f, g], [i, j, k] ]
(33.0, A5DB)
LBL U
// DET = A * (F K  G J)  E * (B K  C J) + I * (B G  C F)
RCL F; RCL* K; RCL G; RCL* J; ; RCL* A;
RCL B; RCL* K; RCL C; RCL* J; ; RCL* E; 
RCL B; RCL* G; RCL C; RCL* F; ; RCL* I; +
RTN
V: swap column in x reg (1..3) with [d, h, l]
(24.0, 8BD5)
LBL V
STO i;
RCL D; X<> (i); STO D;
4; STO+ i;
RCL H; X<> (i); STO H;
4; STO+ i;
RCL L; X<> (i); STO L;
RTN
I: input a 3x3 system (also uses lbl J)
lbl I
1.012 STO I
LBL J
INPUT (I)
ISG I
GTO J
RTN
