College Algebra : Solutions and Solution Sets

Study concepts, example questions & explanations for College Algebra

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Solving Equations And Inequallities

Give all real solutions of the following equation:

\(\displaystyle x^{4} +5x^{2} - 36 = 0\)

Possible Answers:

\(\displaystyle \left \{ -3,3\right \}\)

\(\displaystyle \left \{ 2,3\right \}\)

\(\displaystyle \left \{ -3, -2, 2, 3\right \}\)

\(\displaystyle \left \{ -2,2\right \}\)

\(\displaystyle \left \{ -3, -2\right \}\)

Correct answer:

\(\displaystyle \left \{ -2,2\right \}\)

Explanation:

By substituting \(\displaystyle u = x^{2}\) - and, subsequently, \(\displaystyle u^{2} =\left ( x^{2} \right )^{2} = x^{4}\) this can be rewritten as a quadratic equation, and solved as such:

\(\displaystyle x^{4} +5x^{2} - 36 = 0\)

\(\displaystyle u^{2} + 5u - 36 = 0\)

We are looking to factor the quadratic expression as \(\displaystyle (u+?)(u+?)\), replacing the two question marks with integers with product \(\displaystyle -36\) and sum 5; these integers are \(\displaystyle 9,-4\).

\(\displaystyle (u+9)(u-4) = 0\)

Substitute back:

\(\displaystyle (x^{2}+9)(x^{2}-4) = 0\)

The first factor cannot be factored further. The second factor, however, can itself be factored as the difference of squares:

\(\displaystyle (x^{2}+9)(x+2)(x-2) = 0\)

Set each factor to zero and solve:

\(\displaystyle x^{2}+9= 0\) 

\(\displaystyle x^{2}= -9\)

Since no real number squared is equal to a negative number, no real solution presents itself here. 

\(\displaystyle x-2= 0 \Rightarrow x = 2\)

\(\displaystyle x+2 = 0 \Rightarrow x = - 2\)

 

The solution set is \(\displaystyle \left \{ -2, 2\right \}\).

Example Question #2 : Solving Equations And Inequallities

Give all real solutions of the following equation:

\(\displaystyle x^{4} - 13x^{2} + 36 = 0\)

Possible Answers:

\(\displaystyle \left \{ -9,-4,4,9\right \}\)

The equation has no real solutions.

\(\displaystyle \left \{ 4,9\right \}\)

\(\displaystyle \left \{ -3, -2, 2, 3\right \}\)

\(\displaystyle \left \{ 2, 3\right \}\)

Correct answer:

\(\displaystyle \left \{ -3, -2, 2, 3\right \}\)

Explanation:

By substituting \(\displaystyle u = x^{2}\) - and, subsequently, \(\displaystyle u^{2} =\left ( x^{2} \right )^{2} = x^{4}\) this can be rewritten as a quadratic equation, and solved as such:

\(\displaystyle x^{4} - 13x^{2} + 36 = 0\)

\(\displaystyle u^{2} - 13u + 36 = 0\)

We are looking to factor the quadratic expression as \(\displaystyle (u+?)(u+?)\), replacing the two question marks with integers with product 36 and sum \(\displaystyle -13\); these integers are \(\displaystyle -9,-4\).

\(\displaystyle (u-9)(u-4) = 0\)

Substitute back:

\(\displaystyle (x^{2}-9)(x^{2}-4) = 0\)

These factors can themselves be factored as the difference of squares:

\(\displaystyle (x+3)(x-3)(x+2)(x-2) = 0\)

Set each factor to zero and solve:

\(\displaystyle x-3 = 0 \Rightarrow x = 3\)

\(\displaystyle x-2= 0 \Rightarrow x = 2\)

\(\displaystyle x+2 = 0 \Rightarrow x = - 2\)

\(\displaystyle x+3 = 0 \Rightarrow x = - 3\)

The solution set is \(\displaystyle \left \{ -3, -2, 2, 3\right \}\).

Learning Tools by Varsity Tutors