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Intermediate Algebra Chapter 4 Systems of Linear Equations

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Objective Determine if an ordered pair is a solution for a system of equations.

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System of Equations Two or more equations considered simultaneously form a system of equations.

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Checking a solution to a system of equations 1. Replace each variable in each equation with its corresponding value. 2. Verify that each equation is true.

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Graphing Procedure 1. Graph both equations in the same coordinate system. 2. Determine the point of intersection of the two graphs. 3. This point represents the estimated solution of the system of equations.

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Graphing observations Solution is an estimate Lines appearing parallel have to be checked algebraically. Lines appearing to be the same have to be checked algebraically.

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Classifying Systems Meet in Point – Consistent – independent Parallel – Inconsistent – Independent Same – Consistent - Dependent

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Def: Dependent Equations Equations with identical graphs

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Independent Equations Equations with different graphs.

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Algebraic Check Same Line

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Algebraic Check Parallel Lines

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Algebraic Check Meet in a point {(x,y)}

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Calculator Method for Systems Solve each equation for y Input each equation into Y= Graph Set Window Use Cal Intersect

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Calculator Problem

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Calculator Problem 2

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Calculator Problem 3

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Objective Solve a System of Equations using the Substitution Method.

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Substitution Method 1. Solve one equation for one variable 2. In other equation, substitute the expression found in step 1 for that variable. 3. Solve this new equation (1 variable) 4. Substitute solution in either original equation 5. Check solution in original equation.

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Althea Gibson – tennis player “No matter what accomplishments you make, someone helped you.”

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Intermediate Algebra The Elimination Method

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Notes on elimination method Sometimes called addition method Goal is to eliminate on of the variables in a system of equations by adding the two equations, with the result being a linear equation in one variable.

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1. Write both equations in ax + by = c form 2. If necessary, multiply one or both of the equations by appropriate numbers so that the coefficients of one of the variables are opposites.

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Procedure for addition method cont. 3. Add the equations to eliminate a variable. 4. Solve the resulting equation 5. Substitute that value in either of the original equations and solve for the other variable. 6. Check the solution.

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Procedure for addition method cont. Solution could be ordered pair. If a false statement results i.e. 1 = 0, then lines are parallel and solution set is empty set. (inconsistent) If a true statement results i.e. 0 = 0, then lines are same and solution set is the line itself. (dependent)

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Practice Problem Answer {(2,4)}

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Practice Problem Hint: eliminate x first Answer {(-7/2,-4)}

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Practice Problem

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Practice problem

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Special Note on Addition Method Having solved for one variable, one can eliminate the other variable rather than substitute. Useful with fractions as answers.

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Practice Problem – eliminate one variable and than the other Answer: {(8/3,1/3)}

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Confucius “It is better to light one small candle than to curse the darkness.”

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Intermediate Algebra 4.2 Systems Of Equations In Three Variables

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Objective To use algebraic methods to solve linear equations in three variables.

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Def: linear equation in 3 variables is any equation that can be written in the standard form ax + by +cz =d where a,b,c,d are real numbers and a,b,c are not all zero.

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Def: Solution of linear equation in three variables is an ordered triple (x,y,z) of numbers that satisfies the equation.

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Procedure for 3 equations, 3 unknowns 1. Write each equation in the form ax +by +cz=d Check each equation is written correctly. Write so each term is in line with a corresponding term Number each equation

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Procedure continued: 2. Eliminate one variable from one pair of equations using the elimination method. 3. Eliminate the same variable from another pair of equations. Number these equations

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Procedure continued 4. Use the two new equations to eliminate a variable and solve the system. 5. Obtain third variable by back substitution in one of original equations

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Procedure continued Check the ordered triple in all three of the original equations.

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Sample problem 3 equations

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Answer to 3 eqs-3unknowns {(-2,3,1)}

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Bertrand Russell – mathematician (1872-1970) “Mathematics takes us still further from what is human, into the region of absolute necessity, to which not only the actual world, but every possible world, must conform.”

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Cramer’s Rule Objective: Evaluate determinants of 2 x 2 matrices Objective: Solve systems of equations using Cramer’s Rule

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Determinant

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Cramer’s rule intuitive Each denominator, D is the determinant of a matrix containing only the coefficients in the system. To find D with respect to x, we replace the column of s-coefficients in the coefficient matrix with the constants form the system. To find D with respect to y, replace the column of y- coefficients in the coefficient matrix sit the constant terms.

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Sample Problem: Evaluate: Answer = 16

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Sample Cramer’s Rule problem Solve by Cramer’s Rule

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Cramer’s Rule Answer

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Senecca “It is not because things are difficult that we do not dare, it is because we do not dare that they are difficult.”

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Intermediate Algebra 5.5 Applications Objective: Solve application problems using 2 x 2 and 3 x 3 systems.

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Mixture Problems ****Use table or chart Include all units Look back to test reasonableness of answer.

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Sample Problem How many milliliters of a 10% HCl solution and 30% HCl solution must be mixed together to make 200 milliliters of 15% HCl solution?

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Mixture problem equations

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Mixture problem answers 150 mill of 10% sol 50 mill of 30% sol Gives 200 mill of 15% sol

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Distance Problems Include Chart and/or picture Note distance, rate, and time in chart D = RT and T = D/R and R=D/T Include units Check reasonableness of answer.

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Sample Problem To gain strength, a rowing crew practices in a stream with a fairly quick current. When rowing against the stream, the team takes 15 minutes to row 1 mile, whereas with the stream, they row the same mile in 6 minutes. Find the team’s speed in miles per hour in still water and how much the current changes its speed.

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Distance problem equations

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Answer Team row 7 miles per hour in still water Current changes speed by 3 miles per hour

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Joe Paterno – college football coach “The will to win is important but the will to prepare is vital.”

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Chapter 4 Section 2 Copyright © 2011 Pearson Education, Inc.

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