5.13: Deriving and Using the Quadratic Formula
The profit on your school fundraiser is represented by the quadratic expression , where p is your price point. What is your break-even point (i.e., the price point at which you will begin to make a profit)? Hint: Set the equation equal to zero.
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Khan Academy: Quadratic Formula 1
Guidance
The last way to solve a quadratic equation is the Quadratic Formula. This formula is derived from completing the square for the equation (see #13 from the Problem Set in the previous concept). We will derive the formula here.
Investigation: Deriving the Quadratic Formula
Walk through each step of completing the square of .
1. Move the constant to the right side of the equation.
2. “Take out” from everything on the left side of the equation.
3. Complete the square using .
4. Add this number to both sides. Don’t forget on the right side, you need to multiply it by (to account for the outside the parenthesis).
5. Factor the quadratic equation inside the parenthesis and give the right hand side a common denominator.
6. Divide both sides by .
7. Take the square root of both sides.
8. Subtract from both sides to get by itself.
This formula will enable you to solve any quadratic equation as long as you know , and (from ).
Example A
Solve using the Quadratic Formula.
Solution: First, make sure one side of the equation is zero. Then, find and . . Now, plug in the values into the formula and solve for .
Example B
Solve using the Quadratic Formula.
Solution: Let’s get everything onto the left side of the equation.
Now, use and and plug them into the Quadratic Formula.
Example C
Solve by factoring, completing the square, and the Quadratic Formula.
Solution: While it might not look like it, 51 is not a prime number. Its factors are 17 and 3, which add up to 20.
Now, solve by completing the square.
Lastly, let’s use the Quadratic Formula. .
Notice that no matter how you solve this, or any, quadratic equation, the answer will always be the same.
Intro Problem Revisit The break-even point is the point at which the equation equals zero. So use the Quadratic Formula to solve for p .
Now, use and and plug them into the Quadratic Formula.
Therefore, there are two break-even points: .
Guided Practice
1. Solve using the Quadratic Formula.
2. Solve using all three methods.
Answers
1. and
2. Factoring : . The factors of -30 that add up to -1 are -6 and 5. Expand the term.
Complete the square
Quadratic Formula
Explore More
Solve the following equations using the Quadratic Formula.
Choose any method to solve the equations below.
Solve the following equations using all three methods.
- Writing Explain when you would use the different methods to solve different types of equations. Would the type of answer (real or imaginary) help you decide which method to use? Which method do you think is the easiest?
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Description
Learning Objectives
Here you'll derive the Quadratic Formula and use it to solve any quadratic equation.