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**Get out Homework 9-6 Practice B Answers -13.10, 6.10 -2.15, 2.48 **

Lesson 9.6 Solve Quadratic Equations by the Quadratic Formula Essential Question: How do you solve quadratic equations by using the quadratic formula? Day 3 Date: Warm-up: CC.9.12.A.REI.4b Solve quadratic equations by inspection, by taking square roots, completing the square, the quadratic formula and factoring, as appropriate to the initial form of the equation. Recognize when the quadratic formula gives complex solutions. Get out Homework 9-6 Practice B Answers -13.10, 6.10 -2.15, 2.48 -1.82, 2.07 -3.73, -0.27 -3, 4 -2.61, 1.28 -4.61, -1.39 -13.44, 7.44 -2.11, 2.36 -1.45, 1.25 -3/2, 1/3 no solution

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**Standardized Test Practice**

EXAMPLE 1 Standardized Test Practice What are the solutions of 3x2 + 5x = 8? –1 and – A 8 3 B –1 and 8 3 C 1 and – 8 3 D 1 and 8 3 SOLUTION 3x2 + 5x = 8 Write original equation. 3x2 + 5x – 8 = 0 Write in standard form. x = –b b2 – 4ac 2a Quadratic formula

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**Standardized Test Practice**

EXAMPLE 1 Standardized Test Practice Substitute values in the quadratic formula: a = 3, b = 5, and c = –8. x = –5 52 – 4(3)(–8) 2(3) = –5 121 6 Simplify. = –5 ± 11 6 Simplify the square root. The solutions of the equation are –5 + 11 6 = 1 and –5 – 11 = – 8 3 The correct answer is C. ANSWER

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**Solve a quadratic equation**

EXAMPLE 2 Solve a quadratic equation Solve 2x2 – 7 = x. 2x2 – 7 = x Write original equation. 2x2 – x – 7 = 0 Write in standard form. x = b2 – 4ac + – –b 2a Quadratic formula Substitute values in the quadratic formula: a = 2, b = –1, and c = –7. – (–1) – + ( –1)2 – 4(2)(–7) 2(2) = 4 = + – Simplify.

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EXAMPLE 2 Solve a quadratic equation ANSWER The solutions are 57 57 1 + 1 – 2.14 and –1.64. 4 4 CHECK Write the equation in standard form, 2x2 – x – 7 = 0. Then graph the related function y = 2x2 – x – 7. The x-intercepts are about –1.6 and 2.1. So, each solution checks.

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GUIDED PRACTICE for Examples 1and 2 Solve the equation by Quadratic Formula. 1. x2 – 8x + 16 = 0 ANSWER 4 2. 3n2 – 5n = –1 ANSWER 0.23, 1.43 3. 4z2 = 7z + 2 ANSWER 0.25, 2

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**Use the quadratic formula**

EXAMPLE 3 Use the quadratic formula For the period 1971 2001, the number y of films produced in the world can be modeled by the function y = 10x2 94x where x is the number of years since In what year were 4200 films produced? FILM PRODUCTION SOLUTION y = 10x2 – 94x Write function. 4200 = 10x2 – 94x – 3900 Substitute 4200 for y. 0 = 10x2 – 94x – 300 Write in standard form.

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**Use the quadratic formula**

EXAMPLE 3 Use the quadratic formula x = (–94)2 – 4 (10)(–300) –(–94) + – 2(10) Substitute values in the quadratic formula: a = 10, b = –94, and c = –300. 20,836 94 + – 20 = Simplify. The solutions of the equation are: 94 + 20,836 20 12 94 – –3. and ANSWER There were 4200 films produced about 12 years after 1971, or in 1983.

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GUIDED PRACTICE for Example 3 4. WHAT IF? In Example 3, find the year when 4750 films were produced. ANSWER 1986

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To find:

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EXAMPLE 4 Choose a solution method Tell what method you would use to solve the quadratic equation. Explain your choice(s). a. 10x2 – 7 = 0 SOLUTION a. The quadratic equation can be solved using square roots because the equation can be written in the form x2 = d.

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EXAMPLE 4 Choose a solution method Tell what method you would use to solve the quadratic equation. Explain your choice(s). b. x2 + 4x = 0 SOLUTION b. The equation can be solved by factoring because the expression x2 + 4x can be factored easily. Also, the equation can be solved by completing the square because the equation is of the form ax2 + bx + c = 0 where a = 1 and b is an even number.

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EXAMPLE 4 Choose a solution method Tell what method you would use to solve the quadratic equation. Explain your choice(s). c. 5x2 + 9x – 4 = 0 SOLUTION c. The quadratic equation cannot be factored easily, and completing the square will result in many fractions. So, the equation can be solved using the quadratic formula.

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GUIDED PRACTICE for Example 4 Tell what method you would use to solve the quadratic equation. Explain your choice(s). 5. x2 + x – 6 = 0 SOLUTION Factoring because the expression factors easily.

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GUIDED PRACTICE for Example 4 Tell what method you would use to solve the quadratic equation. Explain your choice(s). 6. x2 – 9 = 0 SOLUTION Factoring because the expression factors easily. Using square roots is another option since the equation can be written in the form x2 = d.

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GUIDED PRACTICE for Example 4 Tell what method you would use to solve the quadratic equation. Explain your choice(s). 7. x2 + 6x = 5 SOLUTION Completing the square because the equation is of the form ax2 + bx = c where a = 1 and b is an even number. Another method is the quadratic formula since the equation does not factor easily.

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Classwork/Homework 9.6 Practice B Finish all!

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