Algebra II : Quadratic Equations and Inequalities

Study concepts, example questions & explanations for Algebra II

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Example Questions

Example Question #201 : Quadratic Equations And Inequalities

Find the roots of the following function.

\displaystyle -8x^2-14x-3

Possible Answers:

\displaystyle x=-\frac{1}{2}, -\frac{1}{4}

\displaystyle x=-\frac{3}{2}, -\frac{3}{4}

\displaystyle x=-\frac{3}{2}, -\frac{1}{4}

\displaystyle x=-{3}, -\frac{1}{4}

\displaystyle x=-\frac{3}{2}, -\frac{1}{2}

Correct answer:

\displaystyle x=-\frac{3}{2}, -\frac{1}{4}

Explanation:

First, set the equation equal to zero.

\displaystyle 0=-8x^2-14x-3

From here, factor the equation into two binomials.

\displaystyle 0=(-2x-3)(4x-1)

Now, set each binomial equal to zero and solve for x.

\displaystyle 0=-2x-3 AND \displaystyle 0=4x-1

\displaystyle 3=-2x AND \displaystyle 1=4x

\displaystyle \frac{3}{-2}=x AND   \displaystyle \frac{1}{4}=x

 

Example Question #201 : Quadratic Equations And Inequalities

Solve the equation \displaystyle x^4+2x^3+x^2.

Possible Answers:

\displaystyle x=0; x=-2

\displaystyle x=0; x=1

\displaystyle x=0; x=-\frac{1}{2}

\displaystyle x=0; x=\frac{1}{2}

\displaystyle x=0; x=-1

Correct answer:

\displaystyle x=0; x=-1

Explanation:

Pull out an \displaystyle x^2

\displaystyle x^2(x^2+2x+1)=0

Then factor

\displaystyle x^2(x+1)(x+1)=0

So,

 \displaystyle x^2=0;(x+1)=0

\displaystyle x=0; x=-1

Example Question #201 : Quadratic Equations And Inequalities

Find the roots of the following function:

\displaystyle f(x)=x^2-7x+12

Possible Answers:

\displaystyle x=-3, -4

\displaystyle x=1, 6

\displaystyle x=3, -4

\displaystyle x=-3, 4

\displaystyle x=3, 4

Correct answer:

\displaystyle x=3, 4

Explanation:

To find the roots of this function, we must set it equal to zero and solve for x:

\displaystyle x^2-7x+12=0

Now, we must find two factors of 12 that add together to get -7. These numbers are -4 and -3. 

We can rewrite this function, then, as two binomials multiplied together:

\displaystyle (x-4)(x-3)=0

Now, because this product equals zero, each of the binomials can be set equal to zero:

\displaystyle x-4=0

\displaystyle x=4

\displaystyle x-3=0

\displaystyle x=3

Our two roots are \displaystyle x=3, 4.

 

Example Question #111 : Functions And Graphs

Find the roots, 

\displaystyle x^2 -x -6 = 0

 

Possible Answers:

\displaystyle x =\left\{2, -3\right\}

\displaystyle x =\left\{1, -6\right\}

\displaystyle x=\left\{1,-5\right\}

\displaystyle x =\left\{-1, 5\right\}

\displaystyle x =\left\{-2, 3\right\}  

Correct answer:

\displaystyle x =\left\{-2, 3\right\}  

Explanation:

\displaystyle x^2 -x -6 = 0

This problem could be worked out using the quadratic formula, but in this particular case it's easier to factor the left side. 

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

 and \displaystyle x = 3 are the roots that zero the expression on the left side of the equation. In the graph, the curve - which happens to be a paraboloa - will cross the x-axis at the roots. 

 

 

Problem 12 plot

 

A few more points...

Observe that the coefficient for the \displaystyle \large x term in the original quadratic is the sum of  \displaystyle 2 and \displaystyle -3. Also, the constant term in the originl equation is the product of   \displaystyle 2 and \displaystyle -3. It's a good rule of thumb to look for numbers that will satsify these conditions when you are setting off to solve a quadratic. Observe how this happens, 

\displaystyle \large (x+a)(x+b)

\displaystyle \large = x^2 +ax +bx + ab

\displaystyle \large = x^2 + (a+b)x +ab

 

If you notice this pattern in a quadratic, then factoring is always a faster approach. The quadratic formula will always work too, but may take a little longer.

Unfortunately you will often find that factoring is not an option since you will not always be abe to easily find such a pattern for most quadratics, especially if the roots are not whole number integers, or if one or both of the roots are complex numbers. 

Example Question #201 : Quadratic Equations And Inequalities

Solve the equation

\displaystyle 6+\frac{12}{x^2-1}=\frac{5}{x-1}

Possible Answers:

\displaystyle x=-\frac{1}{2}, \frac{1}{3}

\displaystyle x=\frac{1}{2}, -\frac{1}{3}

\displaystyle x=\frac{1}{2}, \frac{1}{3}

\displaystyle x=-\frac{1}{2}, -\frac{1}{3}

\displaystyle x=\frac{1}{2}, \frac{1}{4}

Correct answer:

\displaystyle x=\frac{1}{2}, \frac{1}{3}

Explanation:

Step 1: Rewrite denominator of 2nd term in factored form

\displaystyle 6+\frac{12}{(x+1)(x-1)}=\frac{5}{x-1}

Step 2: Multiply each term by the common denominator

\displaystyle 6*(x+1)(x-1)+\frac{12*(x+1)*(x-1)}{(x+1)(x-1)}=\frac{5*(x+1)(x-1)}{x-1}

Step 3: Simplify

\displaystyle 6x^2-6+12=5x+5

Step 4: Combine like terms, set equal to zero

\displaystyle 6x^2-5x+1=0

Step 5: Factor & solve

\displaystyle (3x-1)(2x-1)=0

\displaystyle x=\frac{1}{2},\frac{1}{3}

Example Question #361 : Intermediate Single Variable Algebra

Find the roots of the quadratic function:

\displaystyle f(x)=4x^2-3x-1

Possible Answers:

\displaystyle x=1, -\frac{1}{4}

\displaystyle x=1

\displaystyle x=0

\displaystyle x=-\frac{1}{4}

Correct answer:

\displaystyle x=1, -\frac{1}{4}

Explanation:

To find the roots of a quadratic function, we must find the x values where the function is equal to zero. To do this, we must set the function equal to zero:

\displaystyle 4x^2-3x-1=0

Now, we factor:

\displaystyle (4x+1)(x-1)=0

The factoring can be done using a number of methods.

Now, set each binomial equal to zero and solve for x:

\displaystyle x=-\frac{1}{4}, 1

 

Example Question #201 : Quadratic Equations And Inequalities

What are the roots of \displaystyle x^2-4x-21?

Possible Answers:

\displaystyle x=-3,7

\displaystyle x=-3

\displaystyle x=3,7

\displaystyle x=7

Correct answer:

\displaystyle x=-3,7

Explanation:

To find the roots, or solutions, of this quadratic equation, first factor the equation.

When factored, it's

\displaystyle (x-7)(x+3).

Then, set each of those expressions equal to 0 and solve for x.

Your solutions are

\displaystyle x=-3,7.

Example Question #52 : Finding Roots

Find the roots of the equation \displaystyle x^4-6x^3+9x^2

Possible Answers:

\displaystyle x=0,3

\displaystyle x=0

\displaystyle x=0,3,3

\displaystyle x=3,3

\displaystyle x=0,0,3,3

Correct answer:

\displaystyle x=0,0,3,3

Explanation:

\displaystyle x^4-6x^3+9x^2

Pull out an \displaystyle x^2 term

\displaystyle x^2(x^2-6x+9)

Two numbers are needed that add to \displaystyle -6 and multiply to be \displaystyle 9. Guess and check results in \displaystyle -3 and \displaystyle -3.

\displaystyle x^2(x-3)(x-3)

Each term must be set equal to 0 to find the roots.

The polynomial is degree 4 so there are 4 roots. To make the roots easier to find the expression can be written as

\displaystyle xx(x-3)(x-3)

The roots are \displaystyle 0,0,3,3

Example Question #201 : Quadratic Equations And Inequalities

Find the roots for:  \displaystyle y=x^2-17x +52

Possible Answers:

\displaystyle 4,13

\displaystyle \frac{9\pm\sqrt{127}}{2}

\displaystyle \frac{17\pm\sqrt{13}}{4}

\displaystyle -4,-13

Correct answer:

\displaystyle 4,13

Explanation:

In order to find the roots, factorize the quadratic.

The multiple to the integer 52 are:

\displaystyle [1,52] , [2,26], [4,13]

The last set can produce the middle term.

Write the binomials.

\displaystyle y=x^2-17x +52 = (x-4)(x-13)

Setting the equation equal to zero, we have two equations:

\displaystyle x-4=0

\displaystyle x-13=0

Solving for the equations, we will have \displaystyle x=4,13 as the possible roots.

The answer is:  \displaystyle 4,13

Example Question #52 : Solving Quadratic Equations

Which of the following is a possible root of \displaystyle y=2x^2-3x+20?  

Possible Answers:

\displaystyle \frac{3+i\sqrt{71}}{2}

\displaystyle -\frac{5}{2}

\displaystyle \frac{3- i\sqrt{151}}{4}

\displaystyle \frac{3- i\sqrt{151}}{2}

Correct answer:

\displaystyle \frac{3- i\sqrt{151}}{4}

Explanation:

Use the quadratic equation to solve for the roots.

\displaystyle x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}

Substitute the values of the polynomial \displaystyle y=ax^2+bx+c into the equation.

\displaystyle y=2x^2-3x+20

\displaystyle x=\frac{-(-3)\pm\sqrt{(-3)^2-4(2)(20)}}{2(2)}

Simplify the quadratic formula.

\displaystyle x=\frac{+3\pm\sqrt{9-160}}{4} = \frac{3\pm\sqrt{-151}}{4}

Since we have a negative discriminant, we will have complex roots even though there are no real roots.

The roots are:  \displaystyle x=\frac{3\pm i\sqrt{151}}{4}

One of the possible roots given is:  \displaystyle \frac{3- i\sqrt{151}}{4}

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