confidential 1 completing the square completing the square
TRANSCRIPT
CONFIDENTIAL 1
CompletingCompletingthe Squarethe Square
CONFIDENTIAL 2
Warm UpWarm Up
Solve. Round to the nearest hundredth.
1) 12 = 5x2
2) 3x2 - 4 = 15
3) x2 - 7 = 19
CONFIDENTIAL 3
You have solved quadraticequations by isolating x2 and then using square
roots. This method works if the quadratic equation,when written in standard form, is a perfect square.
Completing the SquareCompleting the Square
When a trinomial is a perfect square, there is arelationship between the coefficient of the x-term
and the constant term.
CONFIDENTIAL 4
x2 + 6x + 9 x2 - 8x +16
62
2 -82
2 Divide the coefficient of the x-term by 2, then square the
result to get the constant term.
An expression in the form x2 + bx is not a perfect square. However, you can use the relationship shown above to
add a term to x2 + bx to form a trinomial that is a perfect square. This is called completing the square .
CONFIDENTIAL 5
Completing the SquareCompleting the Square
WORDS NUMBERS ALGEBRA
To complete the square of x2 + bx, add
to the expression. This will form a perfect square
trinomial.
x2 + 6x + __
x2 + 6x +
x2 + 6x + 9
(x + 3)2
x2 + bx + __
x2 + bx +
x2 + bx + 9
(x + b)2
2
62
2 62
2b2
2
CONFIDENTIAL 6
Completing the SquareCompleting the Square
Complete the square to form a perfect square trinomial.
A) x2 + 10x + __
x2 + 10x
B) x2 - 9x + __
x2 + 9x
-92
2102
2= 52 = 25
814
=
x2 + 10x + 25 x2 - 9x + 814
Identify b.
Find b2
2
Add
to the expression
b2
2
CONFIDENTIAL 7
Now you try!
Complete the square to form a perfect square trinomial.
1a. x2 + 12x + __
1b. x2 - 5x + __
1c. 8x + x2 __
CONFIDENTIAL 8
To solve a quadratic equation in the form x2 + bx = c, first complete the
square of x2 + bx. Then you can solve using square roots.
CONFIDENTIAL 9
Solving a Quadratic Equation by Completing the Square
Step1: Write the equation in the form x2 + bx = c.
Step2: Find .
Step3: Complete the square by adding to both sides of the equation.
Step4: Factor the perfect-square trinomial.
Step5: Take the square root of both sides.
Step6: Write two equations, using both the positive and negative square root, and solve each equation.
b2
2
b2
2
CONFIDENTIAL 10
Solving Solving x2 + bx = c by Completing the Square+ bx = c by Completing the Square
Solve by completing the square.
A) x2 + 14x = 15
Step1: x2 + 14x = 15 The equation is in the form x2 + bx = c.
Step2: 142
2 = 72 = 49 Find b2
2
Complete the square.Step3: x2 + 14x + 49 = 15 + 49
Factor and simplify.Step4: (x + 7)2 = 64
Take the square root of both sides.
Step5: x + 7 = ±8
Next page
CONFIDENTIAL 11
Write and solve two equations.
Step6: x + 7 = 8 or x + 7 = -8x = 1 or x = -15
The solutions are 1 and -15.
Check
x2 + 14x = 15
(1)2 + 14(1) 15
1 + 14 15
15 15
x2 + 14x = 15
(-15)2 + 14(-15) 15
225 – 210 15
15 15
CONFIDENTIAL 12
B) x2 - 2x - 2 = 0
Step1: x2 - 2x - 2 = 0 The equation is in the form x2 + bx = c.
Step2: -22
2 = (-1)2 = 1 Find b2
2
Complete the square.Step3: x2 - 2x + 1 = 2 + 1
Factor and simplify.Step4: (x - 1)2 = 3
Take the square root of both sides.
Step5: x - 1 = ±√3
Write and solve two equations.
Step6: x - 1 = √3 or x - 1 = - √3 x = 1 + √3 or x = 1 - √3
The solutions are 1 + √3 and 1 - √3 .
CONFIDENTIAL 13
Now you try!
Solve by completing the square.
2a. x2 + 10x = -9
2b. t2 - 8t - 5 = 0
CONFIDENTIAL 14
Solving aSolving ax2 + bx = c by Completing the Square+ bx = c by Completing the Square
Solve by completing the square.
A) -2x2 + 12x - 20 = 0
Step1: -2x2 + 12x - 20 = 0 2 2 2
Write in the form x2 + bx = c.
Step2: -62
2 = (-3)2 = 9 Find b2
2
Divide by -2 to make a = 1.
x2 - 6x + 10 = 0
x2 - 6x = -10
Complete the square.Step3: x2 - 6x + 9 = -10 + 9
Factor and simplify.Step4: (x - 3)2 = -1
There is no real number whose square is negative, so there are no real solutions.
CONFIDENTIAL 15
B) 3x2- 10x = -3
Step1: 3x2 - 10x = -3 3 3 3
Rewrite using likedenominators.
Find b2
2
Divide by 3 to make a = 1.
x2 - 10x = -1 3
x2 + (-10x) + 1 = 0 3
Step2: -10. 1 3 2
2= = 100 = 25 36 9
-10 6
2
Step3:
x2 + (-10x) + 25 = -9 + 25 3 9 9 9
x2 + (-10x) + 25 = -1 + 25 3 9 9
Complete the square.
Next page
CONFIDENTIAL 16
Write and solve two equations.
Factor and simplify.
Step5: x – 5 = ± 4 3 3
The solutions are 3 and 1. 3
Step4: x – 5 = 16 3 9
2
Step6: x – 5 = - 4 or x – 5 = - 4 3 3 3 3
Take the square root of both sides.
x = 3 or x = 1 3
CONFIDENTIAL 17
Now you try!
Solve by completing the square.
3a. 3x2 - 5x - 2 = 0
3b. 4t2 - 4t + 9 = 0
CONFIDENTIAL 18
Problem-Solving ApplicationProblem-Solving Application
A landscaper is designing a rectangular brick patio. She has enough bricks to cover 144 square feet. She wants the length
of the patio to be 10 feet greater than the width. What dimensions should she use for the patio? Round to the nearest
hundredth of a foot.
There are enough bricks to cover 144 square feet.
One edge of the patio is to be 10 feet longer than the other edge.
Set the formula for the area of a rectangle equal to 144, the area of the patio. Solve the equation.
Let x be the width.
Then x + 10 is the length.
CONFIDENTIAL 19
Use the formula for area of a rectangle, l × w = A
(x + 10 ) x = 144
Step1: x2 + 10x = 144 The equation is in the form x2 + bx = c.
Step2: 102
2 = 52 = 25 Find b2
2
Complete the square.Step3: x2 + 10x + 24 = 144 + 25
Factor and simplify.Step4: (x + 5)2 = 169
Take the square root of both sides.
Step5: x + 5 = ±13
Next page
CONFIDENTIAL 20
Write and solve two equations.
Step6: x + 5 = 13 or x + 5 = -13
x = 8 or x = -18
Negative numbers are not reasonable for length, so x = 8 is the only solution that makes sense.
The width is 8 feet, and the length is 8 + 10, or 18, feet.
The length of the patio is 10 feet greater than the width. Also, 8 (18) = 144.
CONFIDENTIAL 21
Now you try!
Solve using square roots. Check your answer.
4. An architect designs a rectangular room with an area of 400 ft2 . The length is to be 8 ft longer than the width. Find the dimensions of the room. Round
your answers to the nearest tenth of a foot.
CONFIDENTIAL 22
Assessment
1 )x2 + 14x __ +
Complete the square to form a perfect square trinomial.
2) x2 - 4x + __
3) x2 - 3x + __
CONFIDENTIAL 23
Solve by completing the square.
6) x2 + x = 30
5) x2 - 8x = 9
4) x2 + 6x = -5
CONFIDENTIAL 24
9) x2 + 16x = 92
7) x2 + 2x = 21
8) x2 - 10x = -9
Solve by completing the square.
CONFIDENTIAL 25
10) The length of a rectangle is 4 meters longerthan the width. The area of the rectangle is 80 square
meters. Find the length and width. Round your answers to the nearest tenth of a meter.
CONFIDENTIAL 26
Completing the SquareCompleting the Square
WORDS NUMBERS ALGEBRA
To complete the square of x2 + bx, add
to the expression. This will form a perfect square
trinomial.
x2 + 6x + __
x2 + 6x +
x2 + 6x + 9
(x + 3)2
x2 + bx + __
x2 + bx +
x2 + bx + 9
(x + b)2
2
62
2 62
2b2
2
Let’s review
CONFIDENTIAL 27
Completing the SquareCompleting the Square
Complete the square to form a perfect square trinomial.
A) x2 + 10x + __
x2 + 10x
B) x2 - 9x + __
x2 + 9x
-92
2102
2= 52 = 25
814
=
x2 + 10x + 25 x2 - 9x + 814
Identify b.
Find b2
2
Add
to the expression
b2
2
CONFIDENTIAL 28
Solving a Quadratic Equation by Completing the Square
Step1: Write the equation in the form x2 + bx = c.
Step2: Find .
Step3: Complete the square by adding to both sides of the equation.
Step4: Factor the perfect-square trinomial.
Step5: Take the square root of both sides.
Step6: Write two equations, using both the positive and negative square root, and solve each equation.
b2
2
b2
2
CONFIDENTIAL 29
Solving Solving x2 + bx = c by Completing the Square+ bx = c by Completing the Square
Solve by completing the square.
A) x2 + 14x = 15
Step1: x2 + 14x = 15 The equation is in the form x2 + bx = c.
Step2: 142
2 = 72 = 49 Find b2
2
Complete the square.Step3: x2 + 14x + 49 = 15 + 49
Factor and simplify.Step4: (x + 7)2 = 64
Take the square root of both sides.
Step5: x + 7 = ±8
Next page
CONFIDENTIAL 30
Write and solve two equations.
Step6: x + 7 = 8 or x + 7 = -8x = 1 or x = -15
The solutions are 1 and -15.
Check
x2 + 14x = 15
(1)2 + 14(1) 15
1 + 14 15
15 15
x2 + 14x = 15
(-15)2 + 14(-15) 15
225 – 210 15
15 15
CONFIDENTIAL 31
B) x2 - 2x - 2 = 0
Step1: x2 - 2x - 2 = 0 The equation is in the form x2 + bx = c.
Step2: -22
2 = (-1)2 = 1 Find b2
2
Complete the square.Step3: x2 - 2x + 1 = 2 + 1
Factor and simplify.Step4: (x - 1)2 = 3
Take the square root of both sides.
Step5: x - 1 = ±√3
Write and solve two equations.
Step6: x - 1 = √3 or x - 1 = - √3 x = 1 + √3 or x = 1 - √3
The solutions are 1 + √3 and 1 - √3 .
CONFIDENTIAL 32
Solving aSolving ax2 + bx = c by Completing the Square+ bx = c by Completing the Square
Solve by completing the square.
A) -2x2 + 12x - 20 = 0
Step1: -2x2 + 12x - 20 = 0 2 2 2
Write in the form x2 + bx = c.
Step2: -62
2 = (-3)2 = 9 Find b2
2
Divide by -2 to make a = 1.
x2 - 6x + 10 = 0
x2 - 6x = -10
Complete the square.Step3: x2 - 6x + 9 = -10 + 9
Factor and simplify.Step4: (x - 3)2 = -1
There is no real number whose square is negative, so there are no real solutions.
CONFIDENTIAL 33
Problem-Solving ApplicationProblem-Solving Application
A landscaper is designing a rectangular brick patio. She has enough bricks to cover 144 square feet. She wants the length
of the patio to be 10 feet greater than the width. What dimensions should she use for the patio? Round to the nearest
hundredth of a foot.
There are enough bricks to cover 144 square feet.
One edge of the patio is to be 10 feet longer than the other edge.
Set the formula for the area of a rectangle equal to 144, the area of the patio. Solve the equation.
Let x be the width.
Then x + 10 is the length.
CONFIDENTIAL 34
Use the formula for area of a rectangle, l × w = A
(x + 10 ) x = 144
Step1: x2 + 10x = 144 The equation is in the form x2 + bx = c.
Step2: 102
2 = 52 = 25 Find b2
2
Complete the square.Step3: x2 + 10x + 24 = 144 + 25
Factor and simplify.Step4: (x + 5)2 = 169
Take the square root of both sides.
Step5: x + 5 = ±13
Next page
CONFIDENTIAL 35
Write and solve two equations.
Step6: x + 5 = 13 or x + 5 = -13
x = 8 or x = -18
Negative numbers are not reasonable for length, so x = 8 is the only solution that makes sense.
The width is 8 feet, and the length is 8 + 10, or 18, feet.
The length of the patio is 10 feet greater than the width. Also, 8 (18) = 144.
CONFIDENTIAL 36
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