Elimination Using Multiplication
Prime Factors
Equations Involving Rational Exponents
Working with Percentages and Proportions
Rational Expressions
Interval Notation and Graphs
Simplifying Complex Fractions
Dividing Whole Numbers with Long Division
Solving Compound Linear Inequalities
Raising a Quotient to a Power
Solving Rational Equations
Solving Inequalities
Adding with Negative Numbers
Quadratic Inequalities
Dividing Monomials
Using the Discriminant in Factoring
Solving Equations by Factoring
Subtracting Polynomials
Cube Root
The Quadratic Formula
Multiply by the Reciprocal
Relating Equations and Graphs for Quadratic Functions
Multiplying a Polynomial by a Monomial
Calculating Percentages
Solving Systems of Equations using Substitution
Comparing Fractions
Solving Equations Containing Rational Expressions
Factoring Polynomials
Negative Rational Exponents
Roots and Radicals
Intercepts Given Ordered Pairs and Lines
Factoring Polynomials
Solving Linear Inequalities
Mixed Expressions and Complex Fractions
Solving Equations by Multiplying or Dividing
The Addition Method
Finding the Equation of an Inverse Function
Solving Compound Linear Inequalities
Multiplying and Dividing With Square Roots
Exponents and Their Properties
Equations as Functions
Factoring Trinomials
Solving Quadratic Equations by Completing the Square
Dividing by Decimals
Lines and Equations
Simplifying Complex Fractions
Graphing Solution Sets for Inequalities
Standard Form for the Equation of a Line
Checking Division with Multiplication
Elimination Using Addition and Subtraction
Complex Fractions
Multiplication Property of Equality
Solving Proportions Using Cross Multiplication
Product and Quotient of Functions
Quadratic Functions
Solving Compound Inequalities
Operating with Complex Numbers
Equivalent Fractions
Changing Improper Fractions to Mixed Numbers
Multiplying by a Monomial
Solving Linear Equations and Inequalities Graphically
Dividing Polynomials by Monomials
Multiplying Cube Roots
Operations with Monomials
Properties of Exponents
Mixed Numbers and Improper Fractions
Equations Quadratic in Form
Simplifying Square Roots That Contain Whole Numbers
Dividing a Polynomial by a Monomial
Writing Numbers in Scientific Notation
Solutions to Linear Equations in Two Variables
Solving Linear Inequalities
Multiplying Two Mixed Numbers with the Same Fraction
Special Fractions
Solving a Quadratic Inequality
Parent and Family Graphs
Solving Equations with a Fractional Exponent
Evaluating Trigonometric Functions
Solving Equations Involving Rational Expressions
Laws of Exponents
Multiplying Polynomials
Vertical Line Test
Solving Inequalities with Fractions and Parentheses
Multiplying Polynomials
Solving Quadratic and Polynomial Equations
Extraneous Solutions
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The Addition Method

The addition method can be used to eliminate any variable whose coefficients are opposites. If neither variable has coefficients that are opposites, then we use the multiplication property of equality to change the coefficients of the variables, as shown in Examples 1 and 2.


Example 1

Using multiplication and addition

Solve the system by the addition method:

2x - 3y = -13

5x - 12y = -46


If we multiply both sides of the first equation by -4, the coefficients of y will be 12 and -12, and y will be eliminated by addition.

(-4)(2x - 3y) = (-4)(-13) Multiply each side by -4
5x - 12y = -46  
-8x + 12y = 52  
5x - 12y = -46   Add
-3x   = 6  
x = -2  

Replace x by -2 in one of the original equations to find y:

2x - 3y = -13
2(-2) - 3y = -13
-4 - 3y = -13
-3y = -9
y = 3

Because 2(-2) - 3(3) = -13 and 5(-2) - 12(3) = -46 are both true, the solution set is {(-2, 3)}.


Example 2

Multiplying both equations before adding

Solve the system by the addition method:

-2x + 3y = 6

3x - 5y = -11


To eliminate x, we multiply the first equation by 3 and the second by 2:

3(-2x + 3y) = 3(6) Multiply each side by 3.
2(3x - 5y) = 2(-11) Multiply each side by 2.
-6x + 9y = 18  
6x - 10y = -22   Add.
-y = -4  
y = 4  

Note that we could have eliminated y by multiplying by 5 and 3. Now insert y = 4 into one of the original equations to find x:

-2x + 3(4) = 6 Let y = 4 in -2x + 3y = 6.
-2x + 12 = 6  
-2x = -6  
x = 3  

Check that (3, 4) satisfies both equations. The solution set is {(3, 4)}.

We can always use the addition method as long as the equations in a system are in the same form.

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