Download Elimination

System of Linear Equations
Illustrating Elimination
Solving Systems of Equations by
Elimination
Step 1
Write the system so that like
terms are aligned.
Step 2
Eliminate one of the variables and
solve for the other variable.
Step 3
Substitute the value of the variable
into one of the original equations
and solve for the other variable.
Step 4
Write the answers from Steps 2 and 3
as an ordered pair, (x, y), and check.
Example 1: Elimination Using Addition
Solve
3x – 4y = 10
by elimination.
x + 4y = –2
Step 1
Step 2
3x – 4y = 10
x + 4y = –2
4x + 0 = 8
4x = 8
4x = 8
4
4
x=2
Write the system so that like
terms are aligned.
Add the equations to
eliminate the y-terms.
Simplify and solve for x.
Divide both sides by 4.
Example 1 Continued
Step 3 x + 4y = –2
2 + 4y = –2
–2
–2
4y = –4
4y
–4
4
4
y = –1
Step 4 (2, –1)
Write one of the original
equations.
Substitute 2 for x.
Subtract 2 from both sides.
Divide both sides by 4.
Write the solution as an
ordered pair.
Example 2: Elimination Using Subtraction
Solve
2x + y = –5 by elimination.
2x – 5y = 13
2x + y = –5
–(2x – 5y = 13)
2x + y = –5
–2x + 5y = –13
Step 2
0 + 6y = –18
6y = –18
y = –3
Step 1
Add the opposite of each
term in the second
equation.
Eliminate the x term.
Simplify and solve for y.
Example 2 Continued
Step 3 2x + y = –5
2x + (–3) = –5
2x – 3 = –5
+3
+3
2x
= –2
x = –1
Step 4 (–1, –3)
Write one of the original
equations.
Substitute –3 for y.
Add 3 to both sides.
Simplify and solve for x.
Write the solution as an
ordered pair.
Example 3A: Elimination Using Multiplication First
Solve the system by elimination.
x + 2y = 11
–3x + y = –5
Step 1
Step 2
x + 2y = 11
–2(–3x + y = –5)
x + 2y = 11
+(6x –2y = +10)
7x + 0 = 21
7x = 21
x=3
Multiply each term in the second
equation by –2 to get opposit
y-coefficients.
Add the new equation to
the first equation.
Simplify and solve for x.
Example 3A Continued
Step 3 x + 2y = 11
3 + 2y = 11
–3
–3
2y = 8
y=4
Step 4
(3, 4)
Write one of the original
equations.
Substitute 3 for x.
Subtract 3 from each side.
Simplify and solve for y.
Write the solution as an
ordered pair.
Example 3B: Elimination Using Multiplication First
Solve the system by elimination.
–5x + 2y = 32
2x + 3y = 10
Step 1
2(–5x + 2y = 32)
5(2x + 3y = 10)
–10x + 4y = 64
+(10x + 15y = 50)
Step 2
19y = 114
y=6
Multiply the first equation
by 2 and the second
equation by 5 to get
opposite x-coefficients
Add the new equations.
Simplify and solve for y.
Example 3B Continued
Step 3
2x + 3y = 10
2x + 3(6) = 10
2x + 18 = 10
–18 –18
Step 4
2x = –8
x = –4
(–4, 6)
Write one of the original
equations.
Substitute 6 for y.
Subtract 18 from both sides.
Simplify and solve for x.
Write the solution as an
ordered pair.
 x  2 y  10

2 x  5 y  52
Find a constant that you can multiply the first
equation that will get you the opposite of -2x.
2* ( x  2 y  10)

 2 x  4 y  20
(6,  8)
Elimination
x  2(8)  10
x 16  10
x  10  16
 2 x  4 y  20

2 x  5 y  52
9 y  72
x6
y  8
Lesson Check
Solve each system by elimination.
1.
2x + y = 25
3y = 2x – 13
2.
–3x + 4y = –18
x = –2y – 4
(2, –3)
3.
–2x + 3y = –15
3x + 2y = –23
(–3, –7)
(11, 3)