Solving Systems of Equations

The Elimination Method

Objectives

• Learn the procedure of the Elimination Method using addition

• Learn the procedure of the Elimination Method using multiplication

• Solving systems of equations using the Elimination Method

Elimination using Addition

Consider the system

x - 2y = 5

2x + 2y = 7

REMEMBER: We are trying to find the Point of Intersection. (x, y)

Lets add both equations to each other

Elimination using Addition

Consider the system

x - 2y = 5

2x + 2y = 7

Lets add both equations to each other+

NOTE: We use the Elimination Method, if we can immediately cancel out two like terms.

Elimination using Addition

Consider the system

x - 2y = 5

2x + 2y = 7

Lets add both equations to each other+

3x = 12x = 4

ANS: (4, y)

NOTE: We use the Elimination Method, if we can immediately cancel out two like terms.

Elimination using Addition

Consider the system

x - 2y = 5

2x + 2y = 7

ANS: (4, y)

Lets substitute x = 4 into this equation.

4 - 2y = 5 Solve for y - 2y = 1

y = 12

NOTE: We use the Elimination Method, if we can immediately cancel out two like terms.

Elimination using Addition

Consider the system

x - 2y = 5

2x + 2y = 7

ANS: (4, )

Lets substitute x = 4 into this equation.

4 - 2y = 5 Solve for y - 2y = 1

y = 12

12

NOTE: We use the Elimination Method, if we can immediately cancel out two like terms.

Elimination using Addition

Consider the system

3x + y = 14

4x - y = 7

NOTE: We use the Elimination Method, if we can immediately cancel out two like terms.

Elimination using Addition

Consider the system

3x + y = 14

4x - y = 7

7x = 21x = 3

ANS: (3, y)

+

Elimination using Addition

Consider the system

ANS: (3, )

3x + y = 14

4x - y = 7

Substitute x = 3 into this equation

3(3) + y = 149 + y = 14

y = 5

5

NOTE: We use the Elimination Method, if we can immediately cancel out two like terms.

Examples…

2x y+ 5=

3x y− 15=

1. 2.

2y x− 5=

6y x+ 11=

ANS: (4, -3) ANS: (-1, 2)

Elimination using Multiplication

Consider the system

6x + 11y = -5

6x + 9y = -3

Elimination using Multiplication

Consider the system

6x + 11y = -5

6x + 9y = -3+12x + 20y = -8 When we add equations together,

nothing cancels out

Elimination using Multiplication

Consider the system

6x + 11y = -5

6x + 9y = -3

Elimination using Multiplication

Consider the system

6x + 11y = -5

6x + 9y = -3

-1 ( )

Elimination using Multiplication

Consider the system

- 6x - 11y = 5

6x + 9y = -3+-2y = 2

y = -1

ANS: (x, )-1

Elimination using Multiplication

Consider the system

6x + 11y = -5

6x + 9y = -3

ANS: (x, )-1

y = -1

Lets substitute y = -1 into this equation

6x + 9(-1) = -36x + -9 = -3

+9 +9

6x = 6x = 1

Elimination using Multiplication

Consider the system

6x + 11y = -5

6x + 9y = -3

ANS: ( , )-1

y = -1

Lets substitute y = -1 into this equation

6x + 9(-1) = -36x + -9 = -3

+9 +9

6x = 6x = 1

1

Elimination using Multiplication

Consider the system

x + 2y = 6

3x + 3y = -6

Multiply by -3 to eliminate the x term

Elimination using Multiplication

Consider the system

x + 2y = 6

3x + 3y = -6

-3 ( )

Elimination using Multiplication

Consider the system

-3x + -6y = -18

3x + 3y = -6+-3y = -24

y = 8

ANS: (x, 8)

Elimination using Multiplication

Consider the system

x + 2y = 6

3x + 3y = -6

ANS: (x, 8)

Substitute y =14 into equation

y =8

x + 2(8) = 6x + 16 = 6

x = -10

Elimination using Multiplication

Consider the system

x + 2y = 6

3x + 3y = -6

ANS: ( , 8)

Substitute y =14 into equation

y =8

x + 2(8) = 6x + 16 = 6

x = -10

-10

Examples

1.x + 2y = 5

2x + 6y = 12

2.

ANS: (3, 1)

x + 2y = 4

x - 4y = 16

ANS: (8, -2)

More complex ProblemsConsider the system

3x + 4y = -25

2x - 3y = 6

Multiply by 2

Multiply by -3

More complex ProblemsConsider the system

3x + 4y = -25

2x - 3y = 6

2( )

-3( )

More complex ProblemsConsider the system

6x + 8y = -50

-6x + 9y = -18+17y = -68

y = -4

ANS: (x, -4)

More complex ProblemsConsider the system

3x + 4y = -25

2x - 3y = 6

ANS: (x, -4)

Substitute y = -4

2x - 3(-4) = 62x - -12 = 6

2x + 12 = 6

2x = -6

x = -3

More complex ProblemsConsider the system

3x + 4y = -25

2x - 3y = 6

ANS: ( , -4)

Substitute y = -4

2x - 3(-4) = 62x - -12 = 6

2x + 12 = 6

2x = -6

x = -3 -3

Examples…

1. 2.

4x + y = 9

3x + 2y = 8

2x + 3y = 1

5x + 7y = 3

ANS: (2, 1) ANS: (2, -1)