Algebra II : Parabolic Functions

Study concepts, example questions & explanations for Algebra II

varsity tutors app store varsity tutors android store

Example Questions

Example Question #921 : Algebra Ii

Write a quadratic equation having \displaystyle \left ( -3,2 \right ) as the vertex (vertex form of a quadratic equation).

Possible Answers:

\displaystyle \left ( x - 3 \right )^{2}

\displaystyle \left ( x + 3 \right )^{2} + 2

\displaystyle \left ( x + 3 \right )^{2}

\displaystyle \left ( x + 3 \right )^{2} + 11

\displaystyle \left ( x - 3 \right )^{2} -2

Correct answer:

\displaystyle \left ( x + 3 \right )^{2} + 2

Explanation:

The vertex form of a quadratic equation is given by

\displaystyle \left ( x - h \right )^{2} + k

Where the vertex is located at \displaystyle \left ( h,k \right )

giving us \displaystyle \left ( x + 3 \right )^{2} +2.

Example Question #1 : Quadratic Functions

What are the \displaystyle x-intercepts of the equation?

\displaystyle y=\frac{x^2-9}{x^2-16}

Possible Answers:

\displaystyle x=3

\displaystyle x=4,-4

There are no \displaystyle x-intercepts.

\displaystyle x=2

\displaystyle x=3,-3

Correct answer:

\displaystyle x=3,-3

Explanation:

To find the x-intercepts of the equation, we set the numerator equal to zero.

\displaystyle 0=x^2-9

\displaystyle 9=x^2

\displaystyle \sqrt{9}=\sqrt{x^2}

\displaystyle 3,-3=x

Example Question #2 : Parabolic Functions

What is the minimum possible value of the expression below?

\displaystyle 3x^{2}+6x-10

Possible Answers:

\displaystyle -19

The expression has no minimum value.

\displaystyle -13

\displaystyle -7

\displaystyle -10

Correct answer:

\displaystyle -13

Explanation:

We can determine the lowest possible value of the expression by finding the \displaystyle y-coordinate of the vertex of the parabola graphed from the equation \displaystyle y=3x^{2}+6x-10. This is done by rewriting the equation in vertex form.

\displaystyle 3x^{2}+6x-10

\displaystyle 3(x^{2}+2x)-10

\displaystyle 3(x^{2}+2x+1)-3* 1-10

\displaystyle 3(x+1)^{2}-13

The vertex of the parabola \displaystyle y=3(x+1)^{2}-13 is the point \displaystyle (-1, -13).

The parabola is concave upward (its quadratic coefficient is positive), so \displaystyle -13 represents the minimum value of \displaystyle y. This is our answer.

Example Question #924 : Algebra Ii

Find the coordinates of the vertex of this quadratic function:

\displaystyle y=-2x^{2}+6x-5

Possible Answers:

\displaystyle \left(\frac{3}{2},-\frac{1}{2}\right)

\displaystyle \left(-\frac{3}{2}, -\frac{1}{2}\right)

\displaystyle \left(\frac{2}{3}, -\frac{1}{2}\right)

\displaystyle \left(-\frac{2}{3}, -\frac{1}{2}\right)

\displaystyle \left(\frac{3}{2}, \frac{1}{2}\right)

Correct answer:

\displaystyle \left(\frac{3}{2},-\frac{1}{2}\right)

Explanation:

Vertex of quadratic equation \displaystyle y=ax^{2}+bx+c is given by \displaystyle (h,k).

\displaystyle h=-\frac{b}{2a}, k=c-\frac{b^{2}}{4a}

For \displaystyle y=-2x^{2}+6x-5,

\displaystyle h=-\frac{6}{2\times (-2)}=\frac{3}{2},

\displaystyle k=-5-\frac{6^{2}}{4\times (-2)}=-5+\frac{9}{2}=-\frac{1}{2},

so the coordinate of vertex is \displaystyle \left(\frac{3}{2}, -\frac{1}{2}\right).

Example Question #2 : Parabolic Functions

What are the x-intercepts of the graph of \displaystyle y=8x^{2}+2x-3 ? 

Possible Answers:

\displaystyle -\frac{1}{2}\ and -\frac{3}{4}

\displaystyle -\frac{1}{2}\ and\ \frac{3}{4}

\displaystyle \frac{1}{2} \ and -\frac{3}{4}

\displaystyle -\frac{3}{2}\ and\ -\frac{1}{4}

\displaystyle -\frac{3}{2}\ and\ \frac{1}{4}

Correct answer:

\displaystyle \frac{1}{2} \ and -\frac{3}{4}

Explanation:

Assume y=0,

\displaystyle y=8x^{2}+2x-3=0

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

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

Example Question #2 : Quadratic Functions

Find the vertex of the parabola given by the following equation:

\displaystyle f(x)=-2x^2-12x-23

Possible Answers:

\displaystyle (2,4)

\displaystyle (1,-7)

\displaystyle (-4,12)

\displaystyle (4,1)

\displaystyle (-3,-5)

Correct answer:

\displaystyle (-3,-5)

Explanation:

In order to find the vertex of a parabola, our first step is to find the x-coordinate of its center. If the equation of a parabola has the following form:

\displaystyle f(x)=ax^2+bx+c

Then the x-coordinate of its center is given by the following formula:

\displaystyle x_{center}=-\frac{b}{2a}

For the parabola described in the problem, a=-2 and b=-12, so our center is at:

\displaystyle x_{center}=-\frac{-12}{2(-2)}=-3

Now that we know the x-coordinate of the parabola's center, we can simply plug this value into the function to find the y-coordinate of the vertex:

\displaystyle f(-3)=-2(-3)^2-12(-3)-23=-5

So the vertex of the parabola given in the problem is at the point \displaystyle (-3,-5)

Example Question #1 : Quadratic Functions

Give the minimum value of the function \displaystyle f(x) = 2 x^{2} - 5x + 9.

Possible Answers:

\displaystyle 9

\displaystyle 6

\displaystyle 5\frac{7}{8}

\displaystyle 18\frac{3}{8}

This function does not have a minimum.

Correct answer:

\displaystyle 5\frac{7}{8}

Explanation:

This is a quadratic function. The \displaystyle x-coordinate of the vertex of the parabola can be determined using the formula \displaystyle x = - \frac{b}{2a}, setting \displaystyle a = 2,b = -5:

\displaystyle x = - \frac{b}{2a} = - \frac{-5}{2 \cdot 2} =\frac{5}{4}

Now evaluate the function at \displaystyle x = \frac{5}{4}:

\displaystyle f(x) = 2 x^{2} - 5x + 9

\displaystyle f \left ( \frac{5}{4} \right ) = 2 \left ( \frac{5}{4} \right )^{2} - 5 \left ( \frac{5}{4} \right )+ 9

\displaystyle = 2 \left (\frac{25}{16} \right ) - 5 \left ( \frac{5}{4} \right )+ 9

\displaystyle = \frac{25}{8} - \frac{25}{4} + 9

\displaystyle = \frac{25}{8} - \frac{50}{8} + \frac{72}{8} =\frac{47}{8} =5\frac{7}{8}

Example Question #2 : Parabolic Functions

What is the equation of a parabola with vertex \displaystyle (4,1) and \displaystyle y-intercept \displaystyle (0,-7)?

Possible Answers:

\displaystyle y = -\frac{1}{2} x^{2} +4x - 7

\displaystyle y = \frac{1}{2} x^{2} +8x - 7

\displaystyle y = \frac{1}{2} x^{2} -8x - 7

\displaystyle y = \frac{1}{2} x^{2} -4x - 7

\displaystyle y = -\frac{1}{2} x^{2} -4x - 7

Correct answer:

\displaystyle y = -\frac{1}{2} x^{2} +4x - 7

Explanation:

From the vertex, we know that the equation of the parabola will take the form \displaystyle y = a (x - 4) ^{2}+ 1 for some \displaystyle a .

To calculate that \displaystyle a, we plug in the values from the other point we are given, \displaystyle x = 0, y=-7, and solve for \displaystyle a:

\displaystyle y = a (x - 4)^{2} + 1

\displaystyle -7= a \cdot (0 - 4)^{2} + 1

\displaystyle -7= a \cdot (- 4)^{2} + 1

\displaystyle -7= 16a + 1

\displaystyle 16a = -8

\displaystyle a = -\frac{1}{2}

Now the equation is \displaystyle y = -\frac{1}{2} (x - 4)^{2} + 1. This is not an answer choice, so we need to rewrite it in some way.

Expand the squared term:

\displaystyle y = -\frac{1}{2} (x^{2} -8x + 16) + 1

Distribute the fraction through the parentheses:

\displaystyle y = -\frac{1}{2} x^{2} +4x - 8 + 1

Combine like terms:

\displaystyle y = -\frac{1}{2} x^{2} +4x - 7

Example Question #7 : Quadratic Functions

Determine the maximum or minimum of \displaystyle y=-5x^2+25x+5.

Possible Answers:

Correct answer:

Explanation:

To find the max or min of \displaystyle y=-5x^2+25x+5, use the vertex formula and substitute the appropriate coefficients.

\displaystyle x=\frac{-b}{2a}=\frac{-25}{2(-5)}= \frac{25}{10}=\frac{5}{2}

Since the leading coefficient of \displaystyle -5x^2 is negative, the parabola opens down, which indicates that there will be a maximum.

The answer is:  

Example Question #3 : Parabolic Functions

Factorize:  \displaystyle 6x^2-17x+10

Possible Answers:

\displaystyle (2x-5)(3x-2)

\displaystyle (6x-5)(x-2)

\displaystyle (2x-10)(3x-1)

\displaystyle (6x-5)(x+2)

\displaystyle -(6x+5)(x+2)

Correct answer:

\displaystyle (6x-5)(x-2)

Explanation:

To simplify \displaystyle 6x^2-17x+10, determine the factors of the first and last term.

The factor possibilities of \displaystyle 6x^2:

\displaystyle x, 2x, 3x, 6x

The factor possibilities of \displaystyle 10:

\displaystyle 1,2,5,10

Determine the signs.  Since there is a positive ending term and a negative middle term, the signs of the binomials must be both negative.  Write the pair of parenthesis.

\displaystyle (a -b)(c-d) = ac-ad-bc-bd

These factors must be manipulated by trial and error to determine the middle term.

The correct selection is \displaystyle x, 6x,2, 5.

The  answer is \displaystyle (6x-5)(x-2)

Learning Tools by Varsity Tutors