Rewriting Quadratic Expressions in Factored Form (Part 1)
Algebra 1
6.1
Warm-up
Puzzles of Rectangles
Here are two puzzles that involve side lengths and areas of rectangles. Can you find the missing area in Figure A and the missing length in Figure B? Be prepared to explain your reasoning.
Figure A
A composite figure made up of a square and 2 rectangles. The square has side lengths of 8 inches. It is 5 inches from the far edge of the second rectangle. The square and first rectangle are 10 inches in height together. The first rectangle is marked with question mark square inches. The second rectangle is 4 inches high and 3 inches wide.
Figure B
A composite figure made up of three rectangles. The first rectangle with an area of 36 square inches is on top of another and is taller than it is wide. The first rectangle is 3 inches from the edge of the second rectangle. The second rectangle with an area of 60 square inches, is wider than it is tall. The width of the second rectangle is 12 inches. The third rectangle of 48 square inches is taller than it is wide. It is 3 inches longer than the second rectangle. The first rectangle is 11 inches from the far edge of the third rectangle.
6.2
Activity
Using Diagrams to Understand Equivalent Expressions
Standards Alignment
Building On
Addressing
Building Toward
A-REI.4.b
Solve quadratic equations by inspection (e.g., for x² = 49), taking square roots, completing the square, the quadratic formula and factoring, as appropriate to the initial form of the equation. Recognize when the quadratic formula gives complex solutions and write them as a ± bi for real numbers a and b.
Compare the diagram that shows and the partially completed diagram for to determine the values of and .
What does the bottom right corner tell you about the connection between the constant term, 8, and the missing parts of the factors, and ?
What is the connection between the linear term, , and the two sections of the diagram with the underlines? How is that connected to and ?
Find values for and , and rewrite in factored form.
Rewrite these quadratic expressions in factored form. As you work, consider how the values of the constant and linear terms help decide the values of and .
6.3
Activity
Let’s Rewrite Some Expressions!
Standards Alignment
Building On
Addressing
A-SSE.2
Use the structure of an expression to identify ways to rewrite it. For example, see x4 — y4 as (x²)² — (y²)², thus recognizing it as a difference of squares that can be factored as (x² — y²)(x² + y²).
Solve quadratic equations by inspection (e.g., for x² = 49), taking square roots, completing the square, the quadratic formula and factoring, as appropriate to the initial form of the equation. Recognize when the quadratic formula gives complex solutions and write them as a ± bi for real numbers a and b.
Each row in the table contains a pair of equivalent expressions.
Complete the table with the missing expressions. If you get stuck, consider drawing a diagram.
factored form
standard form
Student Lesson Summary
Previously, you learned how to expand a quadratic expression in factored form and write it in standard form by applying the distributive property.
For example, to expand , we apply the distributive property to multiply by and 4 by . Then, we apply the property again to multiply by , by 5, 4 by , and 4 by 5.
To keep track of all the products, we could make a diagram like this:
Next, we could write the products of each pair inside the spaces:
The diagram helps us see that is equivalent to , or in standard form, .
The linear term, or the term with a single factor of in the standard form of a quadratic expression, is and has a coefficient of 9, which is the sum of 5 and 4.
The constant term, 20, is the product of 5 and 4.
We can use these observations to reason in the other direction: starting with an expression in standard form and writing it in factored form.
For example, suppose we wish to write in factored form.
Let’s start by creating a diagram and writing in the terms and 24.
We need to think of two numbers that multiply to make 24 and add up to -11.
After some thinking, we see that -8 and -3 meet these conditions. The product of -8 and -3 is 24. The sum of -8 and -3 is -11.
So, written in factored form is .
Glossary
linear term
A linear term of an expression has a variable raised to the first power.
In the expression, , the linear term is .
In the expression, , the linear term is .
In the expression , where , , and are constants, the linear term is .
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Find the area of right triangles, other triangles, special quadrilaterals, and polygons by composing into rectangles or decomposing into triangles and other shapes; apply these techniques in the context of solving real-world and mathematical problems.
Solve quadratic equations by inspection (e.g., for x² = 49), taking square roots, completing the square, the quadratic formula and factoring, as appropriate to the initial form of the equation. Recognize when the quadratic formula gives complex solutions and write them as a ± bi for real numbers a and b.
Use the structure of an expression to identify ways to rewrite it. For example, see x4 — y4 as (x²)² — (y²)², thus recognizing it as a difference of squares that can be factored as (x² — y²)(x² + y²).