Chapter 8
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Chapter 8. Systems of Linear Equations or Simultaneous Equations. What is a linear relation?. A relationship between an independent and a independent variable in which as the independent variable changes the dependent variable changes by a constant amount. Examples:

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Systems of Linear Equations or Simultaneous Equations

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Chapter 8

Chapter 8

Systems of Linear Equations or Simultaneous Equations


What is a linear relation

What is a linear relation?

  • A relationship between an independent and a independent variable in which as the independent variable changes the dependent variable changes by a constant amount.

  • Examples:

  • Profit made based on the number of tickets sold to a dance

  • Constant population growth over time

  • Cost for an appliance repair based on a set fee and an hourly price


A system of linear relations is

A System of Linear Relations is

  • Two relations that represent a comparison with thesameinformation

  • A set of equations with the same variables

  • Two lines in the same coordinate plane


An example of a system of equations

An Example of a System of Equations

  • Text p. 454

  • World Records

  • Let s = swim time

    f = float time

  • Which is independent?

  • An equation to represent the record holders time:

    f + 3f = 44

  • An equation to represent the amount of time available to swim and float:

    f + s = 24


System and the graph

System and the graph

  • S + 3f = 44

  • S + f = 24


Linear relations can be represented as

Linear Relations can be represented as:

  • Graphs

  • Equations

  • Mappings

  • Ordered Pairs

  • Tables


Graphing systems of equations

Graphing Systems of Equations

  • Lines in a plane can :

  • be Parallel…Never intersect

    • These lines will have no solution

  • Intersect at only one point

    • These lines will yield one solution

  • be Co-linear…the same line (one line a scale factor of the other)

    • These lines will have an infinite number of solutions


Examples

Examples:

1.) x + 2y = 1

2x + 5 = y

2.) 3x – y = 2

12x – 4y = 8

3.) x – 2y = 4

x = 2y - 2


Lines that

Lines that…

  • Intersect or are co-linear are said to be consistent because there is at least one ordered pair (point) common to both lines.

    • Co-linear have an infinite number of common points!

  • Are parallel are said to be inconsistent because there is not one point common to both lines


Still more vocab

Still more Vocab….

  • If a system has exactly one solution, it is independent, so…. Intersecting lines are independent!!!

  • If a system has an infinite number of solutionsthen it is dependent ……Co-linear lines are dependent!!!

  • These terms DO NOT apply to Parallel Lines


Still more examples

Still More examples:

1.) y = 3x – 4

y = -3x + 4

2.) x + 2y = 5

2x + 4y = 2

* Check with graphing calculator

3.) y = -6

4x + y = 2

4.) 2x + 3y = 4

-4x – 6y = -8


Problem with graphing

Problem with graphing….

The solution when graphing may not be exact

Example: p. 262 Census problem


Other methods for solving systems of equations

Other methods for solving systems of equations

  • Substitution

  • Elimination

    • Addition and Subtraction

    • Multiplication and Division


How to use substitution

How to use Substitution

  • Solve for one of the variables in one of the two equations

  • Which one??

  • The one with a coefficient of 1 or with the easiest coefficient to solve for

  • Substitute the expression equal to the variable into the other equation and solve for the other variable

  • Use this value to find the value for the original variable.


Examples using substitution

Examples using substitution

1.) x + 4y = 1

2x – 3y = -9

2.) 5/2x + y = 4

5x + 2y = 8

3.) 3x + 4y = 7

3/2x + 2y = 11


But still more examples your favorite

But still more examples….(your favorite!)

EJH Labs needs to make 1000 gallons of a 34% acid solution. The only solutions available are 25% acid and 50% acid. How many gallons of each solution should be mixed to make the 34% solution?

A metal alloy is 25% copper. Another metal alloy is 50% copper. How much of each alloy should be used to make 1000 grams of metal alloy that is 45% copper?


When and how to use elimination

When and How to use Elimination

  • Addition and Subtraction

  • Use this method when one of the variables’ coefficients in the two equations is the same or are additive inverses

  • Add or subtract the equations to eliminate a variable

  • Examples:

    1.) ex p. 469

    2a + 4c = 30

    2a + 2c = 21.5

    2.) 3x – 2y = 4

    4x + 2y = 4


Still more examples1

Still more Examples…

The sum of two numbers is 18. The sum of the greater number and twice the smaller number is 25. Find the numbers.

  • The sum of two numbers Is 27. Their difference is 5. Find the numbers.


And more

And More!!

  • Lena is preparing to take the SATs. She has been taking practice tests for a year and her scores are steadily improving. She always scores about 150 higher on math than she does on verbal. She needs a 1270 to get into the college she has chosen. If she assumes that she will still have that 150 difference between the two tests, what will she have to score on each part?


When and how to use elimination1

When and How to use Elimination

  • Multiplication and Division (Scaling)

  • Use this method when all variables have different coefficients

  • Example p. 475

  • 75p + 30n = 40.05

  • 50p + 60n = 35.10


More examples

More Examples…

1.) 2x + 3y = 5

5x + 4y = 16

2.) 3x + 5y = 11

2x + 3y = 7

3.) 2x – 3y = 8

-5x + 2y = 13


Still more examples2

Still More Examples:

  • A bank teller reversed the digits in the amount of a check and overpaid the customer by $9. The sum of the digits in the two digit amount was 9 Find the amount of the check.

  • Example 2 and 3 p. 477


So which method should you use

So which method should you use??

  • Graphing?

  • Substitution?

  • Elimination?


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