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11.2 Solving Quadratic Equations by Completing the Square

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Objective 1 Solve quadratic equations by completing the square when the coefficient of the second-degree term is 1. Slide

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Solve quadratic equations by completing the square when the coefficient of the second-degree term is 1. The methods studied so far are not enough to solve the equation x 2 + 6x + 7 = 0. If we could write the equation in the form (x + 3) 2 equals a constant, we could solve it with the square root property discussed in Section To do that, we need to have a perfect square trinomial on one side of the equation. Recall from Section 5.4 that th perfect square trinomial has the form x 2 + 6x + 9 can be factored as (x + 3) 2. Slide

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The process of changing the form of the equation from x 2 + 6x + 7 = 0 to (x + 3) 2 = 2 is called completing the square. Completing the square changes only the form of the equation. Completing the square not only provides a method for solving quadratic equations, but also is used in other ways in algebra (finding the coordinates of the center of a circle, finding the vertex of a parabola, and so on). Slide Solve quadratic equations by completing the square when the coefficient of the second-degree term is 1. (cont’d)

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Solve x 2 – 20x + 34 = 0. Solution: Slide Rewriting an Equation to Use the Square Root Property CLASSROOM EXAMPLE 1

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Solution: Solve x 2 + 4x = 1. Slide Completing the Square to Solve a Quadratic Equation CLASSROOM EXAMPLE 2

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Completing the Square To solve ax 2 + bx + c = 0 (a ≠ 0) by completing the square, use these steps. Step 1 Be sure the second-degree (squared) term has coefficient 1. If the coefficient of the squared term is one, proceed to Step 2. If the coefficient of the squared term is not 1 but some other nonzero number a, divide each side of the equation by a. Step 2 Write the equation in correct form so that terms with variables are on one side of the equals symbol and the constant is on the other side. Step 3 Square half the coefficient of the first-degree (linear) term. Slide Solve quadratic equations by completing the square when the coefficient of the second-degree term is 1.

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Completing the Square (continued) Step 4 Add the square to each side. Step 5 Factor the perfect square trinomial. One side should now be a perfect square trinomial. Factor it as the square of a binomial. Simplify the other side. Step 6 Solve the equation. Apply the square root property to complete the solution. Steps 1 and 2 can be done in either order. With some equations, it is more convenient to do Step 2 first. Slide Solve quadratic equations by completing the square when the coefficient of the second-degree term is 1.

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Solve x 2 + 3x – 1 = 0. x 2 + 3x = 1 Completing the square. Add the square to each side. x 2 + 3x + = 1 + Slide CLASSROOM EXAMPLE 3 Solving a Quadratic Equation by Completing the Square (a = 1) Solution:

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Use the square root property. Check that the solution set is Slide CLASSROOM EXAMPLE 3 Solving a Quadratic Equation by Completing the Square (a = 1) (cont’d)

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Objective 2 Solve quadratic equations by completing the square when the coefficient of the second-degree term is not 1. Slide

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Solve quadratic equations by completing the square when the coefficient of the second-degree term is not 1. Solving a Quadratic Equation by Completing the Square Step 1: Be sure the second-degree term has a coefficient of 1. If the coefficient of the second-degree term is 1, go to Step 2. If it is not 1, but some other nonzero number a, divide each side of the equation by a. Step 2: Write in correct form. Make sure that all variable terms are on the one side of the equation and that all constant terms are on the other. Step 3: Complete the square. Take half of the coefficient of the first degree term, and square it. Add the square to both sides of the equation. Factor the variable side and combine like terms on the other side. Step 4: Solve the equation by using the square root property. If the solutions to the completing the square method are rational numbers, the equations can also be solved by factoring. However, the method of completing the square is a more powerful method than factoring because it allows us to solve any quadratic equation. Slide

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Solution: Solve 4x 2 + 8x -21 = 0. Slide Solving a Quadratic Equation by Completing the Square CLASSROOM EXAMPLE 4

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Solve 3x 2 + 6x – 2 = 0. 3x 2 + 6x = 2 Completing the square. Slide CLASSROOM EXAMPLE 5 Solving a Quadratic Equation by Completing the Square (a ≠ 1) Solution:

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Use the square root property. Slide CLASSROOM EXAMPLE 5 Solving a Quadratic Equation by Completing the Square (a ≠ 1) (cont’d) Check that the solution set is

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Complete the square. Slide CLASSROOM EXAMPLE 6 Solve for Nonreal Complex Solutions Solve the equation. Solution:

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Slide CLASSROOM EXAMPLE 6 Solve for Nonreal Complex Solutions (cont’d) The solution set is

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Objective 3 Simplify the terms of an equation before solving. Slide

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Solve (x + 6)(x + 2) = 1. Solution: Slide Simplifying the Terms of an Equation before Solving CLASSROOM EXAMPLE 7

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