Download Solve Systems with Elimination

Objective
The student will be able to:
solve systems of equations using
elimination with addition and subtraction.
Solving Systems of Equations
So far, we have solved systems using
graphing and substitution. These notes
show how to solve the system
algebraically using ELIMINATION with
addition and subtraction.
 Elimination is easiest when the
equations are in standard form.
Standard Form: ax + by = c

Solving a system of equations by elimination
using addition and subtraction.
Step 1: Put the equations in
Standard Form.
Step 2: Determine which
variable to eliminate.
Standard Form: Ax + By = C
Look for variables that have the
same coefficient.
Step 3: Add or subtract the
equations.
Solve for the variable.
Step 4: Plug back in to find
the other variable.
Substitute the value of the variable
into the equation.
Step 5: Check your
solution.
Substitute your ordered pair into
BOTH equations.
1) Solve the system using elimination.
x+y=5
3x – y = 7
Step 1: Put the equations in
Standard Form.
Step 2: Determine which
variable to eliminate.
Step 3: Add or subtract the
equations.
They already are!
The y’s have the same
coefficient.
Add to eliminate y.
x+ y=5
(+) 3x – y = 7
4x
= 12
x=3
1) Solve the system using elimination.
x+y=5
3x – y = 7
Step 4: Plug back in to find
the other variable.
Step 5: Check your
solution.
x+y=5
(3) + y = 5
y=2
(3, 2)
(3) + (2) = 5
3(3) - (2) = 7
The solution is (3, 2). What do you think the answer
would be if you solved using substitution?
2) Solve the system using elimination.
4x + y = 7
4x – 2y = -2
Step 1: Put the equations in
Standard Form.
They already are!
Step 2: Determine which
variable to eliminate.
The x’s have the same
coefficient.
Step 3: Add or subtract the
equations.
Subtract to eliminate x.
4x + y = 7
(-) 4x – 2y = -2
3y = 9
Remember to
“keep-changey=3
change”
2) Solve the system using elimination.
4x + y = 7
4x – 2y = -2
Step 4: Plug back in to find
the other variable.
Step 5: Check your
solution.
4x + y = 7
4x + (3) = 7
4x = 4
x=1
(1, 3)
4(1) + (3) = 7
4(1) - 2(3) = -2
Which step would eliminate a variable?
1.
2.
3.
4.
3x + y = 4
3x + 4y = 6
Isolate y in the first
equation
Add the equations
Subtract the equations
Multiply the first
equation by -4
Solve using elimination.
2x – 3y = -2
x + 3y = 17
1.
2.
3.
4.
(2, 2)
(9, 3)
(4, 5)
(5, 4)
3) Solve the system using elimination.
y = 7 – 2x
4x + y = 5
Step 1: Put the equations in
Standard Form.
2x + y = 7
4x + y = 5
Step 2: Determine which
variable to eliminate.
The y’s have the same
coefficient.
Step 3: Add or subtract the
equations.
Subtract to eliminate y.
2x + y = 7
(-) 4x + y = 5
-2x = 2
x = -1
2) Solve the system using elimination.
y = 7 – 2x
4x + y = 5
Step 4: Plug back in to find
the other variable.
Step 5: Check your
solution.
y = 7 – 2x
y = 7 – 2(-1)
y=9
(-1, 9)
(9) = 7 – 2(-1)
4(-1) + (9) = 5
What is the first step when solving with
elimination?
1.
2.
3.
4.
5.
6.
Add or subtract the equations.
Plug numbers into the
equation.
Solve for a variable.
Check your answer.
Determine which variable to
eliminate.
Put the equations in standard
form.
Find two numbers whose sum is 18
and whose difference 22.
1.
2.
3.
4.
14 and 4
20 and -2
24 and -6
30 and 8
Day 2
Solving Systems of Equations
So far, we have solved systems using
graphing, substitution, and elimination.
These notes go one step further and
show how to use ELIMINATION with
multiplication.
 What happens when the coefficients are
not the same?
 We multiply the equations to make them
the same! You’ll see…

Solving a system of equations by elimination
using multiplication.
Step 1: Put the equations in
Standard Form.
Standard Form: Ax + By = C
Step 2: Determine which
variable to eliminate.
Look for variables that have the
same coefficient.
Step 3: Multiply the
equations and solve.
Solve for the variable.
Step 4: Plug back in to find
the other variable.
Step 5: Check your
solution.
Substitute the value of the variable
into the equation.
Substitute your ordered pair into
BOTH equations.
1) Solve the system using elimination.
2x + 2y = 6
3x – y = 5
Step 1: Put the equations in
Standard Form.
Step 2: Determine which
variable to eliminate.
They already are!
None of the coefficients are the
same!
Find the least common multiple
of each variable.
LCM = 6x, LCM = 2y
Which is easier to obtain?
2y
(you only have to multiply
the bottom equation by 2)
1) Solve the system using elimination.
2x + 2y = 6
3x – y = 5
Step 3: Multiply the
equations and solve.
Multiply the bottom equation by 2
2x + 2y = 6
2x + 2y = 6
(2)(3x – y = 5) (+) 6x – 2y = 10
8x
= 16
x=2
Step 4: Plug back in to find
the other variable.
2(2) + 2y = 6
4 + 2y = 6
2y = 2
y=1
1) Solve the system using elimination.
2x + 2y = 6
3x – y = 5
Step 5: Check your
solution.
(2, 1)
2(2) + 2(1) = 6
3(2) - (1) = 5
Solving with multiplication adds one
more step to the elimination process.
2) Solve the system using elimination.
x + 4y = 7
4x – 3y = 9
Step 1: Put the equations in
Standard Form.
Step 2: Determine which
variable to eliminate.
They already are!
Find the least common multiple
of each variable.
LCM = 4x, LCM = 12y
Which is easier to obtain?
4x
(you only have to multiply
the top equation by -4 to
make them inverses)
2) Solve the system using elimination.
x + 4y = 7
4x – 3y = 9
Step 3: Multiply the
equations and solve.
Multiply the top equation by -4
(-4)(x + 4y = 7)
-4x – 16y = -28
4x – 3y = 9) (+) 4x – 3y = 9
-19y = -19
y=1
Step 4: Plug back in to find
the other variable.
x + 4(1) = 7
x+4=7
x=3
2) Solve the system using elimination.
x + 4y = 7
4x – 3y = 9
Step 5: Check your
solution.
(3, 1)
(3) + 4(1) = 7
4(3) - 3(1) = 9
What is the first step when solving with
elimination?
1.
2.
3.
4.
5.
6.
7.
Add or subtract the equations.
Multiply the equations.
Plug numbers into the equation.
Solve for a variable.
Check your answer.
Determine which variable to
eliminate.
Put the equations in standard form.
Which variable is easier to eliminate?
3x + y = 4
4x + 4y = 6
1.
2.
3.
4.
x
y
6
4
3) Solve the system using elimination.
3x + 4y = -1
4x – 3y = 7
Step 1: Put the equations in
Standard Form.
Step 2: Determine which
variable to eliminate.
They already are!
Find the least common multiple
of each variable.
LCM = 12x, LCM = 12y
Which is easier to obtain?
Either! I’ll pick y because the
signs are already opposite.
3) Solve the system using elimination.
3x + 4y = -1
4x – 3y = 7
Step 3: Multiply the
equations and solve.
Multiply both equations
(3)(3x + 4y = -1)
9x + 12y = -3
(4)(4x – 3y = 7) (+) 16x – 12y = 28
25x
= 25
x=1
Step 4: Plug back in to find
the other variable.
3(1) + 4y = -1
3 + 4y = -1
4y = -4
y = -1
3) Solve the system using elimination.
3x + 4y = -1
4x – 3y = 7
Step 5: Check your
solution.
(1, -1)
3(1) + 4(-1) = -1
4(1) - 3(-1) = 7
What is the best number to multiply the top
equation by to eliminate the x’s?
3x + y = 4
6x + 4y = 6
1.
2.
3.
4.
-4
-2
2
4
Solve using elimination.
2x – 3y = 1
x + 2y = -3
1.
2.
3.
4.
(2, 1)
(1, -2)
(5, 3)
(-1, -1)
Find two numbers whose sum is 18
and whose difference 22.
1.
2.
3.
4.
14 and 4
20 and -2
24 and -6
30 and 8