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10.10: Solutions Using the Discriminant

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What if you were given a quadratic equation like x^2 - 3x + 1 = 0 ? How could you determine how many real solutions it had without actually solving it? After completing this Concept, you'll be able to find and interpret the discriminant of a quadratic equation like this one.

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CK-12 Foundation: 1010S The Discriminant

Guidance

In the quadratic formula, x=\frac{-b \pm \sqrt{b^2-4ac}}{2a} , the expression inside the square root is called the discriminant. The discriminant can be used to analyze the types of solutions to a quadratic equation without actually solving the equation. Here’s how:

  • If b^2-4ac>0 , the equation has two separate real solutions.
  • If b^2-4ac<0 , the equation has only non-real solutions.
  • If b^2-4ac=0 , the equation has one real solution, a double root.

Find the Discriminant of a Quadratic Equation

To find the discriminant of a quadratic equation we calculate D=b^2-4ac .

Example A

Find the discriminant of each quadratic equation. Then tell how many solutions there will be to the quadratic equation without solving.

a) x^2-5x+3=0

b) 4x^2-4x+1=0

c) -2x^2+x=4

Solution

a) Plug a = 1, \ b = -5 and c = 3 into the discriminant formula: D=(-5)^2-4(1)(3)=13 D > 0 , so there are two real solutions.

b) Plug a = 4, \ b = -4 and c = 1 into the discriminant formula: D=(-4)^2-4(4)(1)=0 D = 0 , so there is one real solution.

c) Rewrite the equation in standard form: -2x^2+x-4=0

Plug a = -2, \ b = 1 and c = -4 into the discriminant formula: D=(1)^2-4(-2)(-4)=-31 D < 0 , so there are no real solutions.

Interpret the Discriminant of a Quadratic Equation

The sign of the discriminant tells us the nature of the solutions (or roots) of a quadratic equation. We can obtain two distinct real solutions if D > 0 , two non-real solutions if D < 0 or one solution (called a double root) if D = 0 . Recall that the number of solutions of a quadratic equation tells us how many times its graph crosses the x- axis. If D > 0 , the graph crosses the x- axis in two places; if D = 0 it crosses in one place; if D < 0 it doesn’t cross at all:

Example B

Determine the nature of the solutions of each quadratic equation.

a) 4x^2-1=0

b) 10x^2-3x=-4

c) x^2-10x+25=0

Solution

Use the value of the discriminant to determine the nature of the solutions to the quadratic equation.

a) Plug a = 4, \ b = 0 and c = -1 into the discriminant formula: D=(0)^2-4(4)(-1)=16

The discriminant is positive, so the equation has two distinct real solutions.

The solutions to the equation are: \frac{0 \pm \sqrt{16}}{8}=\pm \frac{4}{8}=\pm \frac{1}{2}

b) Re-write the equation in standard form: 10x^2-3x+4=0

Plug a = 10, \ b = -3 and c = 4 into the discriminant formula: D=(-3)^2-4(10)(4)=-151

The discriminant is negative, so the equation has two non-real solutions.

c) Plug a = 1, \ b = -10 and c = 25 into the discriminant formula: D=(-10)^2-4(1)(25)=0

The discriminant is 0, so the equation has a double root.

The solution to the equation is: \frac{10 \pm \sqrt{0}}{2}=\frac{10}{2}=5

If the discriminant is a perfect square, then the solutions to the equation are not only real, but also rational. If the discriminant is positive but not a perfect square, then the solutions to the equation are real but irrational.

Example C

Determine the nature of the solutions to each quadratic equation.

a) 2x^2+x-3=0

b) 5x^2-x-1=0

Solution

Use the discriminant to determine the nature of the solutions.

a) Plug a = 2, \ b = 1 and c = -3 into the discriminant formula: D=(1)^2-4(2)(-3)=25

The discriminant is a positive perfect square, so the solutions are two real rational numbers.

The solutions to the equation are: \frac{-1 \pm \sqrt{25}}{4}=\frac{-1 \pm 5}{4} , so x = 1 and x=-\frac{3}{2} .

b) Plug a = 5, \ b = -1 and c = -1 into the discriminant formula: D=(-1)^2-4(5)(-1)=21

The discriminant is positive but not a perfect square, so the solutions are two real irrational numbers.

The solutions to the equation are: \frac{1 \pm \sqrt{21}}{10} , so x \approx 0.56 and x \approx -0.36 .

Solve Real-World Problems Using Quadratic Functions and Interpreting the Discriminant

You’ve seen that calculating the discriminant shows what types of solutions a quadratic equation possesses. Knowing the types of solutions is very useful in applied problems. Consider the following situation.

Example D

Marcus kicks a football in order to score a field goal. The height of the ball is given by the equation y=-\frac{32}{6400}x^2+x . If the goalpost is 10 feet high, can Marcus kick the ball high enough to go over the goalpost? What is the farthest distance that Marcus can kick the ball from and still make it over the goalpost?

Solution

Define: Let y = height of the ball in feet.

Let x = distance from the ball to the goalpost.

Translate: We want to know if it is possible for the height of the ball to equal 10 feet at some real distance from the goalpost.

Solve:

&\text{Write the equation in standard form:} && -\frac{32}{6400}x^2+x-10= 0\\&\text{Simplify:} && -0.005x^2+x-10 = 0\\&\text{Find the discriminant:} && D =(1)^2-4(-0.005)(-10)=0.8

Since the discriminant is positive, we know that it is possible for the ball to go over the goal post, if Marcus kicks it from an acceptable distance x from the goalpost.

To find the value of x that will work, we need to use the quadratic formula:

x=\frac{-1 \pm \sqrt{0.8}}{-0.01}=189.4 \ feet \ \text{or} \ 10.56 \ feet

What does this answer mean? It means that if Marcus is exactly 189.4 feet or exactly 10.56 feet from the goalposts, the ball will just barely go over them. Are these the only distances that will work? No; those are just the distances at which the ball will be exactly 10 feet high, but between those two distances, the ball will go even higher than that. (It travels in a downward-opening parabola from the place where it is kicked to the spot where it hits the ground.) This means that Marcus will make the goal if he is anywhere between 10.56 and 189.4 feet from the goalposts.

Watch this video for help with the Examples above.

CK-12 Foundation: 1010 The Discriminant

Vocabulary

In the quadratic formula, x=\frac{-b \pm \sqrt{b^2-4ac}}{2a} , the expression inside the square root is called the discriminant. The discriminant can be used to analyze the types of solutions to a quadratic equation without actually solving the equation. Here’s how:

  • If b^2-4ac>0 , the equation has two separate real solutions.
  • If b^2-4ac<0 , the equation has only non-real solutions.
  • If b^2-4ac=0 , the equation has one real solution, a double root.

Guided Practice

Emma and Bradon own a factory that produces bike helmets. Their accountant says that their profit per year is given by the function P=-0.003x^2+12x+27760 , where x is the number of helmets produced. Their goal is to make a profit of $40,000 this year. Is this possible?

Solution

We want to know if it is possible for the profit to equal $40,000.

40000=-0.003x^2+12x+27760

Write the equation in standard form: -0.003x^2+12x-12240=0

Find the discriminant: D=(12)^2-4(-0.003)(-12240)=-2.88

Since the discriminant is negative, we know that it is not possible for Emma and Bradon to make a profit of $40,000 this year no matter how many helmets they make.

Explore More

Find the discriminant of each quadratic equation.

  1. 2x^2-4x+5=0
  2. x^2-5x=8
  3. 4x^2-12x+9=0
  4. x^2+3x+2=0
  5. x^2-16x=32
  6. -5x^2+5x-6=0
  7. x^2+4x=2
  8. -3x^2+2x+5=0

Determine the nature of the solutions of each quadratic equation.

  1. -x^2+3x-6=0
  2. 5x^2=6x
  3. 41x^2-31x-52=0
  4. x^2-8x+16=0
  5. -x^2+3x-10=0
  6. x^2-64=0
  7. 3x^2=7
  8. x^2+30+225=0

Without solving the equation, determine whether the solutions will be rational or irrational.

  1. x^2=-4x+20
  2. x^2+2x-3=0
  3. 3x^2-11x=10
  4. \frac{1}{2}x^2+2x+\frac{2}{3}=0
  5. x^2-10x+25=0
  6. x^2=5x
  7. 2x^2-5x=12
  8. Marty is outside his apartment building. He needs to give his roommate Yolanda her cell phone but he does not have time to run upstairs to the third floor to give it to her. He throws it straight up with a vertical velocity of 55 feet/second. Will the phone reach her if she is 36 feet up? (Hint: the equation for the height is y = -32t^2 + 55t + 4 .)
  9. Bryson owns a business that manufactures and sells tires. The revenue from selling the tires in the month of July is given by the function R=x(200-0.4x) where x is the number of tires sold. Can Bryson’s business generate revenue of $20,000 in the month of July?

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At Grade

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Date Created:

Oct 01, 2012

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Oct 28, 2014
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