1. Objectives
Solve quadratic equations by completing the square.
Write quadratic equations in vertex form.
Read as “plus or minus square root of a.”
Reading Math

2. Write each function in vertex form and identify its vertex.
Notes
1. Complete the square for the expression x2 – 15x Write the resulting expression as a binomial squared.
Solve each equation.
3. x2 = 4x+1
2. x2 – 16x + 64 = 20
Write each function in vertex form and identify its vertex.
5. f(x) = x2 – 12x – 27
4. f(x)= x2 + 6x – 7 2

3. Example 1A: Solving Equations by Using the Square Root Property
Solve the equation.
4x = 59
Subtract 11 from both sides.
4x2 = 48
x2 = 12
Divide both sides by 4 to isolate the square term.
Take the square root of both sides.
Simplify.

4. Example 1B Solve the equation. x2 + 8x + 16 = 49 (x + 4)2 = 49
Factor the perfect square trinomial.
(x + 4)2 = 49
Take the square root of both sides.
x = –4 ±
Subtract 4 from both sides.
x = –11, 3
Simplify.

5. Example 1C Solve the equation. x2 + 12x + 36 = 28 (x + 6)2 = 28
Factor the perfect square trinomial
Take the square root of both sides.
Subtract 6 from both sides.
Simplify.

6. If a quadratic expression of the form x2 + bx cannot model a square, you can add a term to form a perfect square trinomial. This is called completing the square.

7. Example 2A: Solving a Quadratic Equation by Completing the Square
Solve the equation by completing the square.
x2 = 12x – 20
Collect variable terms on one side.
x2 – 12x = –20
Set up to complete the square.
x2 – 12x = –20 +
Add to both sides.
x2 – 12x + 36 = –
Simplify.

8. Example 2A Continued
(x – 6)2 = 16
Factor.
Take the square root of both sides.
Simplify.
x – 6 = ±4
x – 6 = 4 or x – 6 = –4
Solve for x.
x = 10 or x = 2

9. Example 2B
Solve the equation by completing the square.
3x2 – 24x = 27
Divide both sides by 3.
x2 – 8x = 9
Set up to complete the square.
x2 –8x = 9 +
Add to both sides.
Simplify.

10. Example 2B Continued
Solve the equation by completing the square.
Factor.
Take the square root of both sides.
x – 4 =–5 or x – 4 = 5
Solve for x.
x =–1 or x = 9

11. Example 2B imaginary version
Solve the equation by completing the square.
(x – 4)2 = -25
Factor.
Take the square root of both sides.
x – 4 = 5i or x – 4 =-5i
Solve for x.
x =4 + 5i or 4 – 5i 11

12. Recall the vertex form of a quadratic function from lesson 5-1: f(x) = a(x – h)2 + k, where the vertex is (h, k).
You can complete the square to rewrite any quadratic function in vertex form.
In Example 3, the equation was balanced by adding to both sides. Here, the equation is balanced by adding and subtracting on one side.
Helpful Hint

13. Example 3A: Writing a Quadratic Function in Vertex Form
Write the function in vertex form, and identify its vertex.
f(x) = x2 + 16x – 12
Set up to complete the square.
f(x)=(x2 + 16x + ) – 12 –
Add and subtract
f(x) = (x + 8)2 – 76
Simplify and factor.
Because h = –8 and k = –76, the vertex is (–8, –76).

14. Example 3A Continued
Check Use the axis of symmetry formula to confirm vertex.
y = f(–8) = (–8)2 + 16(–8) – 12 = –76 

15. Example 3B
Write the function in vertex form, and identify its vertex
f(x) = x2 + 24x + 145
Set up to complete the square.
f(x) = (x2 + 24x + ) –
Add and subtract
f(x) = (x + 12)2 + 1
Simplify and factor.
Because h = –12 and k = 1, the vertex is (–12, 1).

16. Example 3B Continued
Check Use the axis of symmetry formula to confirm vertex.
y = f(–12) = (–12)2 + 24(–12) = 1 

17. Notes
1. Complete the square for the expression x2 – 15x Write the resulting expression as a binomial squared.
Solve each equation.
3. x2 = 4x+1
2. x2 – 16x + 64 = 20
Write each function in vertex form and identify its vertex.
5. f(x) = x2 – 12x – 27
4. f(x)= x2 + 6x – 7
f(x) = (x + 3)2 – 16; (–3, –16)
f(x) = (x – 6)2 – 36; (6, –36)

18. You can complete the square to solve quadratic equations.