#### Transcript 4.1 Systems of Linear Equations in two variables

Learning Objectives for Section 4.1 Review: Systems of Linear Equations in Two Variables After this lesson, you should be able to solve systems of linear equations in two variables by graphing solve these systems using substitution solve these systems using elimination by addition solve applications of linear systems. 1 System of Linear Equations System of Linear Equations: refers to more than one equation being graphed on the same set of coordinate axes. Solution(s) of a System: where the lines intersect. Also, the solution’s coordinates will work in all of the system’s equations. 2 Solving: Graphically 1. On a single set of coordinate axes, graph each equation and label them. 2. Find the coordinates of the point where the graphs intersect. These coordinates give the solution of the system. Label this point. 3. If the graphs have no point in common, the system has no solution. 4. Check the solution in both of the original equations. 3 Solve by Graphing Example 1: Solve the following system by graphing: y x y 4 2 x y 5 x Check your solution! 4 Checking… x y 4 2 x y 5 Solution: 5 Consistent Systems Consistent system- a system of equations that has a solution. (the system has at least one point of intersection) one solution exists infinitely many solutions exist 6 Inconsistent Systems Inconsistent system- a system of equations that has no solutions. (the lines are parallel) no solution exists 7 Independent and Dependent Equations Independent Equations- the equations graph different lines one solution exists no solution exists Dependent Equations- the equations graph the same line infinitely many solutions exist 8 Solving a System on the Calculator Step 1: Graph each equation. (Use your calculator.) 4x – y = 9 x –3y = 16 *Make sure the equations are in slope-intercept form! Step 2: Find the coordinates of the point of intersection. 2nd CALC 5: intersect First curve? Second curve? (make sure cursor is (make sure cursor on the line for y2.) on the line for y1.) ENTER ENTER Guess? (cursor to where you feel the intersection is) ENTER 9 Calculator Example Continued… 4 x y 9 x 3 y 16 Solution: What type of system is this? What type of equations are these? 10 Example Example: Solve the system by graphing on your calculator. 3x 5 2 y 3x 2 y 7 11 Example with Dependent Equations Example: Solve the system by graphing on your calculator. 5 2 x 3 y 6 y 5 x 2 6 12 Solving: Method of Substitution 1) Solve one of the equations for either x or y. 2) Substitute that result into the other equation to obtain an equation in a single variable (either x or y). 3) Solve the equation for that variable. 4) Substitute this value into any convenient equation to obtain the value of the remaining variable. 5) Check solution in BOTH ORIGINAL equations. 6) Write your solution as an ordered pair. If there is no solution, state 13 that the system is inconsistent. Example Example: Solve the system using substitution. x 3y 9 2 x y 10 14 Another Example Example: Solve the system using substitution. 3x 2 y 7 y 2x 3 15 Solving: Method of Elimination by Addition Method 1) Write both equations in the general form Ax + By = C. 2) Multiply the terms of one or both of the equations by nonzero constants to make the coefficients of one variable differ only in sign. 3) Add the equations and solve for the variable. 4) Substitute the value into one of the ORIGINAL equations to find the value of the other variable. 5) Check the solution in BOTH ORIGINAL equations. 6) Write your solution as an ordered pair. If there is no solution, state that the system is inconsistent. 16 Addition Method Example Example: Solve the system using addition method. 2 y 3x 2 x 7 y 17 Another Addition Method Example Example: Solve the system using addition method. 5 x 16 7 y 2 x 8 y 26 18 Another Example Example: Solve the system using addition method. 2 x 5 y 6 4 x 10 y 1 19 Examples Example: Solve the system using ANY method. x 2 y 8 x 4 2 y 20 Examples Example: Solve the system using ANY method. 2 x 13 y 120 14 x 91y 840 21 Special Cases When solving a system of two linear equations in two variables: If an identity is obtained, such as 0 = 0, then the system has an infinite # of solutions. The equations are dependent The system is consistent. If a contradiction is obtained, such as 0 = 7, then the system has no solution. The system is inconsistent. The equations are independent. 22 Application A man walks at a rate of 3 miles per hour and jogs at a rate of 5 miles per hour. He walks and jogs a total distance of 3.5 miles in 0.9 hours. How long does the man jog? 23 Application A man walks at a rate of 3 miles per hour and jogs at a rate of 5 miles per hour. He walks and jogs a total distance of 3.5 miles in 0.9 hours. How long does the man jog? Solution: Let x represent the amount of time spent walking Let y represent the amount of time spent jogging. 24 Supply and Demand The quantity of a product that people are willing to buy during some period of time depends on its price. Generally, the higher the price, the less the demand; the lower the price, the greater the demand. Similarly, the quantity of a product that a supplier is willing to sell during some period of time also depends on the price. Generally, a supplier will be willing to supply more of a product at higher prices and less of a product at lower prices. The simplest supply and demand model is a linear model where the graphs of a demand equation and a supply equation are straight lines. 25 Supply and Demand (continued) In supply and demand problems we are usually interested in finding the price at which supply will equal demand. This is called the equilibrium price, and the quantity sold at that price is called the equilibrium quantity. If we graph the the supply equation and the demand equation on the same axis, the point where the two lines intersect is called the equilibrium point. Its horizontal coordinate is the value of the equilibrium quantity, and its vertical coordinate is the value of the equilibrium price. 26 Supply and Demand Example Example: Suppose that the supply equation for long-life light bulbs is given by p = 1.04 q - 7.03, and that the demand equation for the bulbs is p = -0.81q + 7.5 where q is in thousands of cases. Find the equilibrium price and quantity, and graph the two equations in the same coordinate system. 27 Supply and Demand (Example continued) If we graph the two equations on a graphing calculator and find the intersection point, we see the graph below. Demand Curve Supply Curve Thus the equilibrium point is (7.854, 1.14), the equilibrium price is $1.14 per bulb, and the equilibrium quantity is 7,854 cases. 28 Now, Another Example A restaurant serves two types of fish dinners- small for $5.99 each and large for $8.99. One day, there were 134 total orders of fish, and the total receipts for these 134 orders was $1024.66. How many small dinners and how many large dinners were ordered? 29 Solution A restaurant serves two types of fish dinners- small for $5.99 each and large for $8.99. One day, there were 134 total orders of fish, and the total receipts for these 134 orders was $1024.66. How many small dinners and how many large dinners were ordered? 30