Lesson 2-3 The Quadratic Equation Objective: To learn the various ways to solve quadratic...
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Lesson 2-3 The Quadratic Equation
Objective: To learn the various ways to solve quadratic
equations, including factoring, completing the square and the
quadratic formula.
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Quadratic Equations A quadratic equation in x is an equation that can
be written in the general form ax2 + bx + c = 0 where a, b, and c are real numbers We can solve by several methods: • By Factoring and setting each factor
equal to 0 • Extracting Square Roots • Completing the Square • Using the Quadratic Formula
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Solving by Factoring Factor ax2 + bx + c =(Ax+B)(Cx+D) = 0
• Set each factor = 0. (Ax+B) = 0, (Cx+D) = 0
• Solve for x
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Solving by Factoring x2 -7x + 12=0 find factors of 12 that
add to -7
(x – 3)(x – 4) = 0
x-3 = 0 x–4 =0 x = 3 x = 4
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Solving by Factoring 2x2 – 7x -15 = 0 (2x + 3)( x- 5) = 0
2x + 3 = 0 x – 5 = 0 2x = -3 x = x = 5
23
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Solving by Factoring 4x2 – 3x = 0
x(4x – 3) = 0
x = 0 4x -3 = 0 4x = 3 x =
43
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Practice 4x2 – x = 0
3x2 – 11x -4 = 0
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Solving by Completing the SquareSolve the following
equation by completing the square:
Step 1: Move quadratic term, and linear term to left side of the equation
2 8 20 0x x
2 8 20x x
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Solving by Completing the Square
Step 2: Find the term that completes the square on the left side of the equation. Add that term to both sides.
2 8 =20 + x x 21 ( ) 4 then square it, 4 16
28
2 8 2016 16x x
2
2
b
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Step 3: Factor the perfect square trinomial on the left side of the equation. Simplify the right side of the equation.
2 8 2016 16x x
Solving by Completing the Square
36)4)(4( xx
36)4( 2 x
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Solving by Completing the Square
Step 4: Take the square root of each side
2( 4) 36x
( 4) 6x
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Solving by Completing the Square
Step 5: Set up the two possibilities and solve
4 64 6 an
d 4 6
10 and 2 x=
xx xx
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Completing the Square-Example #2Solve the following
equation by completing the square:
Step 1: Move quadratic term, and linear term to left side of the equation, the constant to the right side of the equation.
22 7 12 0x x
22 7 12x x
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Solving by Completing the Square
Step 2: Find the term that completes the square on the left side of the equation. Add that term to both sides.
The quadratic coefficient must be equal to 1 before you complete the square, so you must divide all terms by the quadratic coefficient first.
2
2
2
2 7
22 2 2
7 12
72
=-12 +
6
x x
x x
xx
21 7 7 49( ) then square it, 2 62 4 4 1
7
2 49 4916 1
7 62 6
x x
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Solving Completing the Square
Step 3: Factor the perfect square trinomial on the left side of the equation. Simplify the right side of the equation.
2
2
2
7 627 96 494 16 16
7 474
49 4916 1
16
6x x
x
x
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Solving by Completing the Square
Step 4: Take the square root of each side
27 47( )4 16
x
7 47( )4 47 474 47 47
4
x
ix
ix
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Solving by Completing the Square
2
2
2
2
2
1. 2 63 0
2. 8 84 0
3. 5 24 0
4. 7 13 0
5. 3 5 6 0
x x
x x
x x
x x
x x
Try the following examples. Do your work on your paper and then check your answers.
1. 9,72.(6, 14)3. 3,8
7 34.2
5 475.6
i
i
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Warm up Solve by factoring:
x2 + 5x +6=0
2x2 + 9x – 18 = 0
3x2 +x =0
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Taking Square Roots x2 – 3 = 0 (can’t factor) x2 = 3 take the square root of both
sides x = √3 or x = - √3
x = ±√3
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Taking Square Roots x2 + 9 = 0 x2 = -9
x = √-9 x = ±3i
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Taking Square Roots 2(x – 1)2 – 4 = 0 +4 +4 2(x – 1)2 = 4 2 2
(x – 1)2 = 2 take the sq. root of each side x – 1 = ±√2 +1 +1 x = 1±√2
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Practice 5x2 + 13 = 0
(2x – 7)2 – 5 = 0
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The Quadratic Formula2 42
b b acxa
Songx equals negative bplus or minus the square rootof b squared minus 4acall over 2a
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Quadratic FormulaLet’s look at an example.3x2 - 4x + 3 = 0a = ?b = ?c = ?
a = 3b = -4c = 3
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Quadratic FormulaNow let’s plug it in.b = -4, so -b = -(-4) = 4
4 ( 4)2 4(3)(3)2(3)
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Quadratic Formula
Simplify
4 16 366
4 16 366
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Quadratic Formula
Find the zeros ofr2 - 7r -18 = 0
27 ( 7) 4(1)( 18)2(1)
r
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Quadratic FormulaSimplify
7 49 ( 72)2
7 121 7 112 2
r
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Quadratic Formula
Now let’s examine our solution.
We can break this into two equations.
7 112
r
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Quadratic Formula Now we can get our two
solutions.7 11 18 9
2 27 11 4 2
2 2
r
r
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Quadratic Formula x2 – 8x = -10
4x2 -2x +1 = 0