High School Math : Understanding the Discriminant

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #43 : Intermediate Single Variable Algebra

Use the discriminant to determine the nature of the roots:

\(\displaystyle 4x^2-40x+25=0\)

Possible Answers:

\(\displaystyle 2\) rational roots

\(\displaystyle 1\) imaginary root

\(\displaystyle 2\) imaginary roots

\(\displaystyle 2\) irrational roots

\(\displaystyle 1\) rational root

Correct answer:

\(\displaystyle 2\) irrational roots

Explanation:

The formula for the discriminant is:

\(\displaystyle b^2 - 4ac\)

\(\displaystyle =(-40)^2 - 4(4)(25)\)

\(\displaystyle =1200^{}\)

Since the discriminant is positive and not a perfect square, there are \(\displaystyle 2\) irrational roots.

Example Question #12 : Quadratic Equations And Inequalities

Use the discriminant to determine the nature of the roots:

\(\displaystyle 2y^2+6y+5=0\)

Possible Answers:

\(\displaystyle 1\) rational root

\(\displaystyle 1\) imaginary root

\(\displaystyle 2\) imaginary roots

\(\displaystyle 2\) irrational roots

\(\displaystyle 2\) rational roots

Correct answer:

\(\displaystyle 2\) imaginary roots

Explanation:

The formula for the discriminant is:

\(\displaystyle b^2 - 4ac\)

\(\displaystyle =(6)^2 - 4(2)(5)\)

\(\displaystyle =-4\)

Since the discriminant is negative, there are \(\displaystyle 2\) imaginary roots.

Example Question #1 : Understanding The Discriminant

Use the discriminant to determine the nature of the roots:

\(\displaystyle 4x^2-8x+13=0\)

Possible Answers:

\(\displaystyle 1\) imaginary root

Cannot be determined

\(\displaystyle 2\) real roots

\(\displaystyle 1\) real root

\(\displaystyle 2\) imaginary roots

Correct answer:

\(\displaystyle 2\) imaginary roots

Explanation:

The formula for the discriminant is:

\(\displaystyle b^2 - 4ac\)

\(\displaystyle =(-8)^2 - 4(4)(13)\)

\(\displaystyle =-144\) 

Since the discriminant is negative, there are \(\displaystyle 2\) imaginary roots.

Example Question #1 : Discriminants

Given \(\displaystyle 2x^{2}+3x-5=0\) , what is the value of the discriminant?

Possible Answers:

\(\displaystyle 40\)

\(\displaystyle 27\)

\(\displaystyle 49\)

\(\displaystyle 7\)

\(\displaystyle -31\)

Correct answer:

\(\displaystyle 49\)

Explanation:

In general, the discriminant is \(\displaystyle b^{2}-4ac\).

In this particual case \(\displaystyle a=2, b=3\; and\; c=-5\).

Plug in these three values and simplify: \(\displaystyle (3)^{2}-4(2)(-5)= 49\)

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