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Substitution Method: Procedure for Substitution Method 1. Solve one of the equations for one of the variables. 2. Substitute the expression found in step 1 into the other equation. 3. Now solve for the remaining variable. 4. Substitute the value from step 2 into the equation written in step 1, and solve for the remaining variable. Substitution Method: 1. Solve the following system of equations by substitution. y x3 x y 5 Step 2:Substitute x+3 into 2nd equation and solve. x ( x 3) 5 2 x 3 5 2 x 8 x 4 Step 1 is already completed. Step 3: Substitute –4 into 1st equation and solve. y x3 y 4 3 y 1 The answer: ( -4 , -1) 1) Solve the system using substitution x+y=5 y=3+x Step 1: Solve an equation for one variable. Step 2: Substitute Step 3: Solve the equation. The second equation is already solved for y! x+y=5 x + (3 + x) = 5 2x + 3 = 5 2x = 2 x=1 1) Solve the system using substitution x+y=5 y=3+x Step 4: Plug back in to find the other variable. Step 5: Check your solution. x+y=5 (1) + y = 5 y=4 (1, 4) (1) + (4) = 5 (4) = 3 + (1) The solution is (1, 4). What do you think the answer would be if you graphed the two equations? 2) Solve the system using substitution 3y + x = 7 4x – 2y = 0 Step 1: Solve an equation for one variable. It is easiest to solve the first equation for x. 3y + x = 7 -3y -3y x = -3y + 7 Step 2: Substitute 4x – 2y = 0 4(-3y + 7) – 2y = 0 2) Solve the system using substitution 3y + x = 7 4x – 2y = 0 Step 3: Solve the equation. Step 4: Plug back in to find the other variable. -12y + 28 – 2y = 0 -14y + 28 = 0 -14y = -28 y=2 4x – 2y = 0 4x – 2(2) = 0 4x – 4 = 0 4x = 4 x=1 2) Solve the system using substitution 3y + x = 7 4x – 2y = 0 Step 5: Check your solution. (1, 2) 3(2) + (1) = 7 4(1) – 2(2) = 0 Substitution Method Example Solve the system. 3 x 2 y 11 x y 3 Solution y x 3 3 x 2( x 3) 11 3 x 2 x 6 11 5x 5 x 1 y 1 3 y4 (1) (2) Solve (2) for y. Substitute y = x + 3 in (1). Solve for x. Substitute x = 1 in y = x + 3. Solution set: {(1, 4)} Your Turn: 3x – y = 4 x = 4y - 17 Your Turn: 2x + 4y = 4 3x + 2y = 22 Clearing Fractions or Decimals Systems without a Single Point Solution 0 = 4 untrue Inconsistent Systems - how can you tell? An inconsistent system has no solutions. (parallel lines) Substitution Technique ( A) ( B) y 3x 5 y 3x 2 3x 5 3x 2 3x 3x 5 2 2 2 7 0 inconsistent 0 = 0 or n = n Dependent Systems – how can you tell? A dependent system has infinitely many solutions. (it’s the same line!) Substitution Technique ( A) 3y 2x 6 ( B) 12 y 8 x 24 ( B) 8 x 12 y 24 x 32 y 3 ( A) 3 y 2( 32 y 3) 6 3y 3y 6 6 6 6 dependent