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4.6 The Quadratic Formula and the Discriminant

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1 4.6 The Quadratic Formula and the Discriminant
Objectives: Solve quadratic equations by using the Quadratic Formula Use the discriminant to determine the number and type of roots for a quadratic equation.

2 Quadratic Formula Always works to solve a quadratic equation, but is a little lengthy. The solutions of a quadratic equation of the form where a≠0 are given by the formula:

3 Example x²-8x=33 x²-8x-33=0 Set = 0 a=1, b=-8, c=-33

4 Another Example 7x²+6x+2=0 a=7, b=6, c=2 Since ALL of the coefficients are divisible by 2, simplify by dividing them by 2.

5 Discriminant The discriminant describes the solution to a quadratic equation. The part of the quadratic formula under the radical is the discriminant or b²-4ac. If b2 – 4ac > 0, and a perfect square You have two rational roots If b2 – 4ac >0, and not a perfect square. You have two irrational roots If b2 – 4ac = 0 You have 1 real, rational root. (Repeated root) If b2 – 4ac < 0 You have two complex roots

6 Examples b. 7x²-3x=0 (-3)²-4(7)(0)= 9-0=9
Two rational roots because 9 is positive and a perfect square. 3x²-x+5=0 (-1)²-4(3)(5)= 1-60=-59 Two complex roots because the discriminant is a negative. Examples Find the discriminant and describe the number and type of roots. x²-16x+64=0 b²-4ac (-16)²-4(1)(64)= =0 One real, rational root because the discriminant equals 0.

7 We have discussed several methods for solving quadratic equations – which one do you use?
Can Be Used When to Use Graphing sometimes Use only if an exact number is not required. Best use to check the reasonableness of solutions found algebraically Factoring Use if the constant term is 0 or if the factors are easily determined Square Root Property Use for equations in which a perfect square is equal to a constant Completing the Square always Useful for equations of the form ax2 + bx + c where b is even Quadratic Formula When other methods fail or are too tedious

8 Solve – use any method. 4. x²+5x+8=0 Doesn’t factor, not easily done by completing the square (5 is odd) so use quadratic formula. 1. 7x²-14x=0 7x(x-2)=0 x=0, x=2 x²-64=0 x²=64 x=8 x²-16x+64=0 (x-8)(x-8)=0 x=8

9 Homework Workbook Page 55

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