Don’t
be a
hater!
Lesson 4-6: Consistent &
Dependent Systems
Objective:
Students will:
Determine whether a system of equations is
consistent, inconsistent, or dependent.
Consistent Systems
At least 1 solution
Two possibilities:
1) Lines intersect
2) Same line
Inconsistent system
If lines are parallel they are called inconsistent
There is no solution to the system
Lines have same slope but are not same line (different intercepts)
Dependent System: Infinite number of solutions
►lines that are actually the same
►one equation is a scalar of the other
(can multiply by a factor to become the other)
How to find consistent systems: (Ones having 1 or more solutions)
Compare Slopes
►if they are different
► if they are the same & dependent (same line)
Remember in standard form the slope m 
A
B
Example 1 Consistent?
x–y=2
What is the slope?
1
x+y=4
What is the slope?
-1
They do not have the same slope so they are
consistent (only one solution)
Example 2 Consistent?
2x + y = 4
What is the slope?
4x + 2y = 16
What is the slope?
Since the slopes are the same we need to determine if they are the
same line consistent or parallel lines inconsistent
Is one of the equations a scalar of the other?
No, if you multiply the top equation by 2 you get 4x +2y =8 so they
are not the same line and therefore inconsistent (no solution)
-2
-2
Example 3: Consistent or inconsistent for 3x 3:
1) x + 2y + z =1
2) 3x + 3y + z = 2
3) 2x + y = 2
Multiplying 2) by -1 and adding to 1) yields:
-2x – y = -1 What is the slope? -2
Multiplying 3) by -1 yields:
What is the slope?
-2x –y = -2
-2
Since they have the same slope but are not the same
line they are inconsistent
Example 3 Is this system dependent?
x – 3y = 2
3x – 9y = 6
If we multiply the top equation by a scalar of 3 we get:
3x - 9y = 6 which is equation 2!!
The two lines are the same and the system is dependent
(infinite number of solutions)
Make sure both equations are in the same formEither slope-intercept or standard before checking!
3-variable systems
Combine 2 equations to eliminate a variable
If this new equation is a multiple of the 3rd equation →
DEPENDENT
Example 4 Dependent?
x+y=1
-x + z = 1
y+z=2
Which equations are easiest to combine?
1 and 2 to get y + z =2
This is already the 3rd equation!! (scalar of 1) so
dependent