Algebra II : Quadratic Functions

Study concepts, example questions & explanations for Algebra II

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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 #921 : Algebra Ii

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

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

Possible Answers:

There are no \(\displaystyle x\)-intercepts.

\(\displaystyle x=3,-3\)

\(\displaystyle x=2\)

\(\displaystyle x=3\)

\(\displaystyle x=4,-4\)

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 #1 : Quadratic Functions

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

Possible Answers:

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

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

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

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

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

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 #141 : Functions And Lines

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

Possible Answers:

\(\displaystyle 18\frac{3}{8}\)

\(\displaystyle 6\)

\(\displaystyle 9\)

This function does not have a minimum.

\(\displaystyle 5\frac{7}{8}\)

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:

\(\displaystyle \textup{Max at: }x=\frac{5}{2}\)

\(\displaystyle \textup{Max at: }x=-\frac{2}{5}\)

\(\displaystyle \textup{Min at: }x=-\frac{5}{2}\)

\(\displaystyle \textup{Max at: }x=-\frac{5}{2}\)

\(\displaystyle \textup{Min at: }x=\frac{5}{2}\)

Correct answer:

\(\displaystyle \textup{Max at: }x=\frac{5}{2}\)

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:  \(\displaystyle \textup{Max at: }x=\frac{5}{2}\)

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 \(\displaystyle (6x-5)(x-2)\)

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