6-4 solving polynomial equations day 1staffweb.psdschools.org/kemotich/mrs_motichka/algebra...
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64 Solving Polynomial Equations Day 1
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6-4 Solving Polynomial Equations
Objective:Solve cubic polynomial equations by factoring.
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Factor each expression.
1. x2 - 2x - 15 2. x2 - 9x + 14 3. x2 + 6x + 5
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Think back to Ch 5...a quadratic difference of squares has a special factoring pattern.
Think forward to Ch 6...a cubic sum of cubes and difference of cubes also have special factoring patterns.
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Don't believe the pattern?
Multiply to verify...
(a + b)(a2 - ab + b2) = a(a2 - ab + b2) + b(a2 - ab + b2)
= a3 - a2b + ab2 + a2b - ab2 + b3
= a3 + b3
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Factoring a Sum or Difference of Cubes
Example #1: Factor x3 - 8 Example #2: Factor 8x3 + 1
x3 - 8 8x3 + 1
(x)3 - (2)3 (2x)3 + (1)3
(x - 2)(x2 + 2x + (2)2) (2x + 1)((2x)2 - 2x + (1)2)
(x - 2)(x2 + 2x + 4) (2x + 1)(4x2 - 2x + 1)
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Example #3: Solve 27x3 + 1 = 0. Find all complex roots.
27x3 + 1 = (3x)3 + (1)3
= (3x + 1)((3x)2 - 3x + (1)2)
= (3x + 1)(9x2 - 3x + 1)
Since 3x + 1 is a factor, x = -1/3 is a root.
Since 9x2 - 3x + 1 is a factor, we must use the quadratic formula to solve...
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Solve each Equation:
Example #4: x3 + 8 = 0 Example #5: 27x3 - 1 = 0
x3 + 8 = (x)3 + (2)3 27x3 - 1 = (3x)3 - (1)3
= (x + 2)(x2 - 2x + (2)2) = (3x - 1)((3x)2 + 3x + (1)2)
=(x + 2)(x2 - 2x + 4) =(3x - 1)(9x2 + 3x + 1)
x = -2 and x = ? x = 1/3 and x = ?
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Homework:page 330 (12 - 17)
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