Today I shall be dealing with how to solve for 3 unknown in linear algebraic equations using one of the simplest method (Gauss Elimination method) gauss elimination method is one of the oldest, simplest, and most frequently used method for solving systems of algebraic equations
In this method we shall employ the elementary Row operations which include ...
1.. Interchanging any two rows
2.. Multiplying/dividing any row by a nonzero constant.
3.. Adding/subtracting a constant multiple of Row to another Row
Now let's go straight to solving some equation
EXAMPLE 1
Solve the set of linear equations
a + 2b + 3c = 5
3a - b + 2c = 8
4a - 6b + 4c = -2
(Now carefully followup the step by step solution, don't forget to comment on areas you don't understand)
Now using the elementary row operation we will make the first element in R2 (Row 2) which is 3a to be 0
And in R3 (row3) we shall make 4a and -6b to be 0
So that we easily solve for c , b and a
( you will understand once we start implementing this)
Back to our question
a + 2b + 3c = 5
3a - b + 2c = 8
4a - 6b - 4c = -2
Since we want to eliminate 3a in Row 2 (R2), we shall multiply R1 by -3 and add to R2 to form new row 2
( NOTE: our R1 will not be changed but our operation will then become our new row 2)
-3×R1 + R2
-3 × R1 … -3[a+2b+3c=5] == -3a-6b-9c=-15
R2 … 3a-b+2c=8
.•. -3R1 + R2 = (-3a+3a=0), (-6b+-b=-7b), (-9c+2c=-7c),(-15+8=-7)
Now our new equation will look like this
a+2b+3c=5
0-7b-7c=-7. ( note that our R2 changed to our operation)
4a-6b-4c=-2
Now eliminating '4a' in R3
R1×-4 + R3 (because -4 + 4 is zero)
-4×R1…… -4a-8b-12c=-20
+ R3……… 4a-6b-4c=-2
-4R1 + R3 == 0 - 14b - 16c =-22
Our new equation
a+2b+3c=5
0-7b-7c=-7
0-14b-16c=-22
Now we will use row 2 to make -14b in R3 be Zero
-2R2 + R3
-2R2 … 0+14b+14c=14
R3 …….0-14b-16c=-22
-2R2 + R3 = 0+0-2c=-8
Our new equation
a+2b+3c=5
0-7b-7c=-7
0+0-2c=-8
BRAVO!!! We have successfully reduced our equation using the elementary row operation. Now we can easily solve for our c,b and a
From equ.3
0+0-2c=-8
-2c=-8
C= -8/-2
C=4
Now let's substitute C as 4 in equ. 2
0-7b-7c=-7
-7b-7(4)=-7
-7b-28=-7
-7b = -7+28
-7b = 21
b = 21/-7
b = -3
Now let's substitute c as 4 and b as -3 in equ. 1
a+2b+3c=5
a + 2(-3) + 3(4)=5
a-6+12=5
a+6=5
a= 5-6
a=-1
,a=-1, b= -3 and c= 5
So you can see that its so simple, don't think that its too long, its jus because am explaining along..
Once you know how to solve with this method, u will confirm how short it is, compared to other methods...
Pls always solve more related questions and don't forget to log unto this site for help (jus place your questions in the comment sector)
Example 2
Solve the set of linear equation
.a+3b+2c=3
2a-b-3c=-8
5a+2b+c=9
Lets solve
To eliminate 2a from R2 we shall multiply R1 by 2 then add to R1 to form new R2
(Note that am using R as Row)
.•. (R1 × -2) + R1 shall give us a new R2 and make our equation look like thus
a+3b+2c=3
0-7b-7c=-14
5a+2b+c=9
( I hope you understand how I arrived at this,, if no pls comment)
Now let's use R1 to eliminate 5a of R3
R1×-5 + R3
R1×-5…… -5[a+3b+2c=3]
= -5a-15b-10c=-15
R3…… 5a+2b+c=9
R1×-5 + R3
0-13b-9c=-6
Our new equation will then be
.a+3b+2c=3
0-7b-7c=-14
0-13b-9c=-6
Now lets eliminate -13b off equ.3
(Note that we shall use R2 to perform this operation,)
So R2 × 13 + R3 × -7
R2×13... (0×13=0), (-7b×13=-91b), (-7c×13=91c), (-14×13=-182)
R3×-7 ..... 91b+63c=42
R2×13 + R3×-7 = 0+0-28c=-140
Our new equation
.a+3b+2c=3
0-7b-7c=-14
0+0-28c=-140
Now let's solve for our c,b and a
From equ. 3
0+0-28c=-140
-28c=-140
C = -140/-28
C=5
In equ.2 sub. 5 as c
0-7b-7c=-14
-7b-7(5)=-14
-7b-35=-14
-7b=-14+35
-7b=21
.b=21/-7
.b=-3
In equ.1 sub. -3 as b and 5 as c
.a+3b+2c=3
.a = 3 + 9 -10
.a=2