Back to Lesson

Teacher Guide: The Quadratic Formula

Learn to solve any quadratic equation using the quadratic formula.

Present LessonShare to ClassroomView Student Page

Use this lesson with your class

Free, no student accounts needed.

Share with students

Students open the lesson and practise with instant feedback.

Printable worksheet

All practice problems on paper, with a separate answer key.

Worksheet Answer key

Class quiz

10 questions on Quadratic Equations. Students join with a name, you see everyone's score.

For Teachers

Learning Objectives
  • Apply the quadratic formula to solve equations of the form ax2+bx+c=0ax^2 + bx + c = 0
  • Correctly identify the coefficients aa, bb, and cc from any quadratic equation
  • Calculate and interpret the discriminant to predict the number of solutions
  • Simplify solutions including those with irrational numbers
Prerequisites
  • • Solving one-step and two-step equations
  • • Understanding square roots and simplifying radicals
  • • Order of operations (PEMDAS)
  • • Working with negative numbers
Discussion Starters
  • 1. Why do you think mathematicians developed the quadratic formula instead of just using trial and error?
  • 2. Can you think of situations where knowing exactly when something will happen (like a ball landing) is important?
  • 3. If an equation has no real solutions, what does that tell us about the real-world situation it models?
  • 4. How can you tell just by looking at a quadratic equation whether it will be easy or hard to solve?
Common Misconceptions

The quadratic formula only works for certain equations

Remediation: Emphasize that it works for ALL quadratic equations in standard form. Have students verify by trying both factoring and the formula on the same equation.

A negative discriminant means you made a calculation error

Remediation: Show examples where negative discriminants are correct (e.g., x2+1=0x^2 + 1 = 0). Relate to graphs that don't cross the x-axis.

Differentiation Ideas

For Struggling Students:

  • • Provide a formula template with blanks to fill in
  • • Start with equations where a=1a = 1 and bb, cc are small positive integers
  • • Use color-coding to match coefficients to formula positions

For On-Level Students:

  • • Practice with negative coefficients and non-unit leading coefficients
  • • Solve application problems requiring setting up the equation first
  • • Interpret discriminant values before solving

For Advanced Students:

  • • Derive the quadratic formula from completing the square
  • • Explore complex number solutions when discriminant is negative
  • • Analyze how changing coefficients affects the nature of solutions
Standards Alignment
  • A-REI.B.4 (CCSS.MATH.CONTENT.HSA.REI.B.4)

    Solve quadratic equations in one variable using the quadratic formula

  • A-REI.B.4b (CCSS.MATH.CONTENT.HSA.REI.B.4.B)

    Recognize when the quadratic formula gives complex solutions and write them as a±bia \pm bi

Lesson Resources
  • visualDiscriminant Explorer

    Interactive graph showing how discriminant affects parabola position

  • activityFormula Practice

    Step-by-step guided practice with immediate feedback

  • worksheetMixed Practice

    Problems ranging from simple to complex applications

Lesson Content

Everything students see: definition, examples, common mistakes, applications. Tap to open.

Definition

The quadratic formula solves any equation of the form ax2+bx+c=0ax^2 + bx + c = 0:
x=−b±b2−4ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
Where:
  • aa, bb, and cc are the coefficients from the equation
  • a≠0a \neq 0 (otherwise it's not quadratic)
  • The ±\pm symbol means there are usually two solutions
The expression under the square root, b2−4acb^2 - 4ac, is called the discriminant.

Worked Examples

Solve x2+6x+5=0x^2 + 6x + 5 = 0 using the quadratic formula.
1

Identify a, b, and c

Comparing to ax2+bx+c=0ax^2 + bx + c = 0: a=1a = 1, b=6b = 6, c=5c = 5
2

Calculate the discriminant

b2−4ac=62−4(1)(5)=36−20=16b^2 - 4ac = 6^2 - 4(1)(5) = 36 - 20 = 16 → D=16D = 16
3

Apply the formula

x=−6±162(1)=−6±42x = \frac{-6 \pm \sqrt{16}}{2(1)} = \frac{-6 \pm 4}{2} → x=−6±42x = \frac{-6 \pm 4}{2}
4

Find both solutions

x1=−6+42=−22=−1x_1 = \frac{-6 + 4}{2} = \frac{-2}{2} = -1 x2=−6−42=−102=−5x_2 = \frac{-6 - 4}{2} = \frac{-10}{2} = -5 → x=−1x = -1 or x=−5x = -5

Common Mistakes

Forgetting to make b negative when it's already negative

Why it's wrong: If b=−7b = -7, then −b=−(−7)=7-b = -(-7) = 7, not −7-7. The formula says −b-b, so you negate whatever b is.

Correct: Always write −b-b first, then substitute: if b=−7b = -7, write −(−7)=7-(-7) = 7

Calculating b2b^2 incorrectly when b is negative

Why it's wrong: Students sometimes write −72=−49-7^2 = -49 instead of (−7)2=49(-7)^2 = 49

Correct: The entire coefficient is squared: (−7)2=(−7)×(−7)=49(-7)^2 = (-7) \times (-7) = 49

Dividing only part of the numerator by 2a

Why it's wrong: Writing −b2a±b2−4ac\frac{-b}{2a} \pm \sqrt{b^2-4ac} instead of −b±b2−4ac2a\frac{-b \pm \sqrt{b^2-4ac}}{2a}

Correct: The entire numerator (−b±...)(-b \pm \sqrt{...}) is divided by 2a2a

Stopping at one solution

Why it's wrong: The ±\pm means plus OR minus, giving two potential solutions

Correct: Always calculate both: x1=−b+D2ax_1 = \frac{-b + \sqrt{D}}{2a} and x2=−b−D2ax_2 = \frac{-b - \sqrt{D}}{2a}

Why It Matters

The quadratic formula is one of the most powerful tools in algebra because:
  • Universal: It works for ANY quadratic equation, even when factoring doesn't
  • Predictable: Follow the same steps every time
  • Real-world applications: Physics (projectile motion), engineering (parabolic structures), finance (profit optimization)
While factoring is faster when it works, many real-world problems have "messy" numbers that don't factor nicely. The quadratic formula handles them all!

Real World Applications

Projectile Motion

When you throw a ball, its height follows a quadratic equation. The formula helps find when it lands.

Example:

A ball thrown upward has height h=−5t2+20t+1h = -5t^2 + 20t + 1 meters after tt seconds. To find when it hits the ground (h=0h = 0), solve −5t2+20t+1=0-5t^2 + 20t + 1 = 0 using the formula.

1Try It Yourself

A rocket's height is modeled by h=−4t2+32th = -4t^2 + 32t meters.

When does the rocket return to the ground?

Step 1: Write the mathematical expression

Set h=0h = 0 and identify the coefficients:

Business Profit Optimization

Companies use quadratic equations to model profit and find break-even points.

Example:

A company's profit is P=−2x2+100x−800P = -2x^2 + 100x - 800 dollars, where xx is units sold. To find break-even points (where P=0P = 0), use the quadratic formula.

2Try It Yourself

A shop's weekly profit is P=−x2+50x−400P = -x^2 + 50x - 400 dollars for selling xx items.

At what sales levels does the shop break even?

Step 1: Write the mathematical expression

Solve −x2+50x−400=0-x^2 + 50x - 400 = 0:

Key Takeaways

  • 1The quadratic formula is x=−b±b2−4ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
  • 2It works for any equation in the form ax2+bx+c=0ax^2 + bx + c = 0
  • 3The discriminant D=b2−4acD = b^2 - 4ac tells you how many solutions exist
  • 4If D>0D > 0: two distinct real solutions; if D=0D = 0: one repeated solution; if D<0D < 0: no real solutions
  • 5Always check your answers by substituting back into the original equation

Frequently Asked Questions

When should I use the quadratic formula instead of factoring?

Use the quadratic formula when: (1) the equation doesn't factor easily, (2) the coefficients are large or decimals, or (3) you want a reliable method that always works. Factoring is faster when it works, but the formula is more universal.

What does it mean when the discriminant is zero?

When b2−4ac=0b^2 - 4ac = 0, the equation has exactly one solution (called a repeated or double root). Graphically, the parabola just touches the x-axis at one point.

Why is there a plus-minus sign in the formula?

The ±\pm comes from taking the square root. Since both 52=255^2 = 25 and (−5)2=25(-5)^2 = 25, the square root of 25 could be either +5 or -5. This gives us two possible values for x.

Glossary

Quadratic equation
An equation of the form ax2+bx+c=0ax^2 + bx + c = 0 where a≠0a \neq 0
Discriminant
The expression b2−4acb^2 - 4ac that determines the number and type of solutions
Coefficient
The numerical factor of a term (e.g., in 3x23x^2, the coefficient is 3)
Root/Solution
A value of xx that makes the equation true (where the parabola crosses the x-axis)

Formula Card

Quadratic Formula

x=−b±b2−4ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}

Solves any equation $ax^2 + bx + c = 0$

Discriminant

D=b2−4acD = b^2 - 4ac

Determines number and type of solutions

Two solutions

D>0D > 0

Discriminant positive: two distinct real roots

One solution

D=0D = 0

Discriminant zero: one repeated root

No real solutions

D<0D < 0

Discriminant negative: no real roots

More in This Topic

Solving Quadratic Equations by Square Roots

Intermediate25 min

Introduction to Quadratic Equations

Advanced25 min

Vertex Form

Intermediate25 min

Completing the Square

Advanced30 min