Common Core: 8th Grade Math : Expressions & Equations

Study concepts, example questions & explanations for Common Core: 8th Grade Math

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Example Questions

Example Question #91 : Expressions & Equations

The equation of a line is \displaystyle 12x-8y=6. Find the slope of this line.

Possible Answers:

\displaystyle \frac{3}{2}

\displaystyle -\frac{3}{2}

\displaystyle -\frac{3}{4}

\displaystyle \frac{3}{4}

Correct answer:

\displaystyle \frac{3}{2}

Explanation:

To find the slope, you will need to put the equation in \displaystyle y=mx+b form. The value of \displaystyle m will be the slope.

\displaystyle 12x-8y=6

Subtract \displaystyle 6 from either side:

\displaystyle 8y=12x-6

Divide each side by \displaystyle 8:

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

You can now easily identify the value of \displaystyle m.

\displaystyle m=\frac{3}{2}

Example Question #1 : How To Find X Or Y Intercept

What is the \displaystyle y-intercept of the graph of the function \displaystyle f \left ( x\right ) = 2x^{2} - 7x + 5 ?

Possible Answers:

\displaystyle \left (0,5 \right )

\displaystyle \left (0,2 \right )

\displaystyle \left (0,0 \right )

\displaystyle \left ( 0, -3 \frac{1}{2}\right )

\displaystyle \left (0,-7 \right )

Correct answer:

\displaystyle \left (0,5 \right )

Explanation:

The \displaystyle y-intercept of the graph of a function is the point at which it intersects the \displaystyle y-axis - that is, at which \displaystyle x = 0. This point is \displaystyle \left (0, f(0) \right ), so evaluate \displaystyle f(0):

\displaystyle f \left ( x\right ) = 2x^{2} - 7x + 5

\displaystyle f \left (0\right ) = 2 \cdot 0^{2} - 7 \cdot 0 + 5

\displaystyle f \left (0\right ) = 0- 0 + 5 = 5

The \displaystyle y-intercept is \displaystyle \left (0,5 \right ).

Example Question #1 : Use Similar Triangles To Show Equal Slopes: Ccss.Math.Content.8.Ee.B.6

Give the \displaystyle y-intercept, if there is one, of the graph of the equation

\displaystyle y = \frac{1}{x^{2}+ x} + 3

Possible Answers:

\displaystyle (0,4)

\displaystyle (0,3)

The graph has no \displaystyle y-intercept.

\displaystyle (0,2)

\displaystyle \left ( 0, \frac{3}{2}\right )

Correct answer:

The graph has no \displaystyle y-intercept.

Explanation:

The \displaystyle y-intercept is the point at which the graph crosses the \displaystyle y-axis; at this point, the \displaystyle x-coordinate is 0, so substitute \displaystyle 0 for \displaystyle x in the equation:

\displaystyle y = \frac{1}{x^{2}+ x} + 3

\displaystyle y = \frac{1}{0^{2}+0} + 3

\displaystyle y = \frac{1}{0} + 3

However, since this expression has 0 in a denominator, it is of undefined value. This means that there is no value of \displaystyle x paired with \displaystyle y-coordinate 0, and, subsequently, the graph of the equation has no \displaystyle y-intercept.

Example Question #1 : Use Similar Triangles To Show Equal Slopes: Ccss.Math.Content.8.Ee.B.6

Give the \displaystyle y-intercept, if there is one, of the graph of the equation

\displaystyle y = \frac{3}{\left | 3x-8\right |}

Possible Answers:

\displaystyle \left ( 0, -\frac{3}{8}\right )

The graph has no \displaystyle y-intercept.

\displaystyle \left ( 0, \frac{3}{5}\right )

\displaystyle \left ( 0,- \frac{3}{5}\right )

\displaystyle \left ( 0, \frac{3}{8}\right )

Correct answer:

\displaystyle \left ( 0, \frac{3}{8}\right )

Explanation:

The \displaystyle y-intercept is the point at which the graph crosses the \displaystyle y-axis; at this point, the \displaystyle x-coordinate is 0, so substitute \displaystyle 0 for \displaystyle x in the equation:

\displaystyle y = \frac{3}{\left | 3x-8\right |}

\displaystyle y = \frac{3}{\left | 3 \cdot 0 -8\right |}

\displaystyle y = \frac{3}{\left | 0 -8\right |}

\displaystyle y = \frac{3}{\left | -8\right |}

\displaystyle y = \frac{3}{ 8}

The \displaystyle y-intercept is \displaystyle \left ( 0, \frac{3}{8}\right ).

Example Question #131 : Grade 8

Give the \displaystyle y-intercept, if there is one, of the graph of the equation

\displaystyle y = 3(x-2)^{2} + 4 (x-7).

Possible Answers:

The graph does not have a \displaystyle y-intercept.

\displaystyle (0, 8)

\displaystyle (0, -42)

\displaystyle (0, -16)

\displaystyle (0, -64)

Correct answer:

\displaystyle (0, -16)

Explanation:

The \displaystyle y-intercept is the point at which the graph crosses the \displaystyle y-axis; at this point, the \displaystyle x-coordinate is 0, so substitute \displaystyle 0 for \displaystyle x in the equation:

\displaystyle y = 3(x-2)^{2} + 4 (x-7)

\displaystyle y = 3(0-2)^{2} + 4 (0-7)

\displaystyle y = 3( -2)^{2} + 4 ( -7)

\displaystyle y = 3 \cdot 4 + 4 ( -7)

\displaystyle y = 12+ 4 ( -7)

\displaystyle y = 12+ (-28)

\displaystyle y = -16

The \displaystyle y-intercept is the point \displaystyle (0, -16)

Example Question #132 : Grade 8

A line passes through \displaystyle (5, 2) and is perpendicular to the line of the equation \displaystyle 3x-4y = 17. Give the \displaystyle x-intercept of this line.

Possible Answers:

\displaystyle \left (6 \frac{1}{2}, 0 \right )

The line has no \displaystyle x-intercept.

\displaystyle \left (2 \frac{1}{3}, 0 \right )

\displaystyle \left ( 8\frac{2}{3}, 0\right )

\displaystyle \left ( -1\frac{3}{4}, 0\right )

Correct answer:

\displaystyle \left (6 \frac{1}{2}, 0 \right )

Explanation:

First, find the slope of the second line \displaystyle 3x-4y = 17 by solving for \displaystyle y as follows:

\displaystyle 3x-4y = 17

\displaystyle 3x-4y +4y - 17 = 17+4y - 17

\displaystyle 4y = 3x-17

\displaystyle \frac{4y }{4}=\frac{ 3x-17 }{4}

\displaystyle y = \frac{3}{4}x- \frac{17}{4}

The equation is now in the slope-intercept form \displaystyle y = mx+ b; the slope of the second line is the \displaystyle x-coefficient \displaystyle m= \frac{3}{4}.

The first line, being perpendicular to the second, has as its slope the opposite of the reciprocal of \displaystyle \frac{3}{4}, which is \displaystyle m=- \frac{4}{3}.

Therefore, we are looking for a line through \displaystyle (5, 2) with slope \displaystyle m=- \frac{4}{3}. Using point-slope form

\displaystyle y - y_{1} = m(x -x_{1})

with 

\displaystyle x _{1 } =5, y _{1 } = 2,m=- \frac{4}{3},

the equation becomes

\displaystyle y - 2 =- \frac{4}{3}(x -5).

To find the \displaystyle x-intercept, substitute 0 for \displaystyle y and solve for \displaystyle x:

\displaystyle 0 - 2 =- \frac{4}{3}(x -5)

\displaystyle - 2 =- \frac{4}{3}x+ \frac{20}{3}

\displaystyle - 2- \frac{20}{3} =- \frac{4}{3}x+ \frac{20}{3} - \frac{20}{3}

\displaystyle - \frac{26}{3} =- \frac{4}{3}x

\displaystyle - \frac{3} {4}\cdot\left (- \frac{26}{3} \right )=- \frac{3} {4}\cdot \left (- \frac{4}{3}x \right )

\displaystyle x= \frac{26}{4} = \frac{13}{2} = 6\frac{1}{2}

The  \displaystyle x-intercept is the point \displaystyle \left (6 \frac{1}{2}, 0 \right ).

Example Question #4 : Use Similar Triangles To Show Equal Slopes: Ccss.Math.Content.8.Ee.B.6

A line passes through \displaystyle (5, 2) and is parallel to the line of the equation \displaystyle 3x-4y = 17. Give the \displaystyle x-intercept of this line.

Possible Answers:

\displaystyle \left ( 8\frac{2}{3}, 0\right )

\displaystyle \left (2 \frac{1}{3}, 0 \right )

\displaystyle \left (6 \frac{1}{2}, 0 \right )

\displaystyle \left ( -1\frac{3}{4}, 0\right )

The line has no \displaystyle x-intercept.

Correct answer:

\displaystyle \left (2 \frac{1}{3}, 0 \right )

Explanation:

First, find the slope of the second line \displaystyle 3x-4y = 17 by solving for \displaystyle y as follows:

\displaystyle 3x-4y = 17

\displaystyle 3x-4y +4y - 17 = 17+4y - 17

\displaystyle 4y = 3x-17

\displaystyle \frac{4y }{4}=\frac{ 3x-17 }{4}

\displaystyle y = \frac{3}{4}x- \frac{17}{4}

The equation is now in the slope-intercept form \displaystyle y = mx+ b; the slope of the second line is the \displaystyle x-coefficient \displaystyle m= \frac{3}{4}.

The first line, being parallel to the second, has the same slope. 

Therefore, we are looking for a line through \displaystyle (5, 2) with slope \displaystyle m= \frac{3}{4}. Using point-slope form

\displaystyle y - y_{1} = m(x -x_{1})

with 

\displaystyle x _{1 } =5, y _{1 } = 2,m= \frac{3}{4},

the equation becomes

\displaystyle y - 2 = \frac{3}{4}(x -5).

To find the \displaystyle x-intercept, substitute 0 for \displaystyle y and solve for \displaystyle x:

\displaystyle 0 - 2 = \frac{3}{4}(x -5)

\displaystyle - 2 = \frac{3}{4} x - \frac{15}{4}

\displaystyle - 2 + \frac{15}{4}= \frac{3}{4} x - \frac{15}{4} + \frac{15}{4}

\displaystyle \frac{3}{4} x = \frac{7}{4}

\displaystyle \frac{4} {3}\cdot \frac{3}{4} x =\frac{4} {3}\cdot \frac{7}{4}

\displaystyle x = \frac{7}{3} = 2\frac{1}{3}

The \displaystyle x-intercept is the point \displaystyle \left (2 \frac{1}{3}, 0 \right ).

Example Question #1 : How To Find X Or Y Intercept

Give the \displaystyle y-intercept of the line with slope \displaystyle \frac{2}{5} that passes through point \displaystyle (5, 9).

Possible Answers:

\displaystyle \left (0, -27 \frac{1}{2} \right )

\displaystyle (0, -11)

The line has no \displaystyle y-intercept.

\displaystyle \left (0, -17 \frac{1}{2} \right )

\displaystyle (0, 7)

Correct answer:

\displaystyle (0, 7)

Explanation:

By the point-slope formula, this line has the equation

\displaystyle y - y_{1} = m(x -x_{1})

where

\displaystyle x_{1} = 5, y_{1} = 9 ,m = \frac{2}{5}

By substitution, the equation becomes

\displaystyle y -9 = \frac{2}{5}(x -5) 

To find the \displaystyle y-intercept, substitute 0 for \displaystyle x and solve for \displaystyle y:

\displaystyle y -9 = \frac{2}{5}(0 -5)

\displaystyle y -9 = \frac{2}{5}( -5)

\displaystyle y - 9 = -2

\displaystyle y = 7

The \displaystyle y-intercept is the point \displaystyle (0, 7).

Example Question #6 : Use Similar Triangles To Show Equal Slopes: Ccss.Math.Content.8.Ee.B.6

Give the \displaystyle y-intercept of the line that passes through points \displaystyle (-3, 4) and \displaystyle (2, -3).

Possible Answers:

\displaystyle \left (0, - \frac{1}{5} \right )

\displaystyle \left (0, - \frac{1}{7} \right )

The line has no \displaystyle y-intercept.

\displaystyle \left (0, - 5\frac{6}{7} \right )

\displaystyle \left (0, - 8 \frac{1}{5} \right )

Correct answer:

\displaystyle \left (0, - \frac{1}{5} \right )

Explanation:

First, find the slope of the line, using the slope formula

\displaystyle m = \frac{y_{2} -y_{1}}{x_{2}-x_{1}}

setting \displaystyle x_{1} = -3, y_{1} = 4, x_{2} = 2, y_{2} = -3:

\displaystyle m = \frac{-3 -4}{2-(-3)} = \frac{-7}{5} =- \frac{7}{5}

By the point-slope formula, this line has the equation

\displaystyle y - y_{1} = m(x -x_{1})

where

\displaystyle x_{1} = -3, y_{1} = 4, m = - \frac{7}{5}; the line becomes

\displaystyle y - 4 = - \frac{7}{5}(x -(-3))

or

\displaystyle y - 4 = - \frac{7}{5}(x +3)

To find the \displaystyle y-intercept, substitute 0 for \displaystyle x and solve for \displaystyle y:

\displaystyle y - 4 = - \frac{7}{5}(0+3)

\displaystyle y - 4 = - \frac{7}{5}(3)

\displaystyle y - 4 = - \frac{21}{5}

\displaystyle y - 4+ 4 = - \frac{21}{5} + 4

\displaystyle y = - \frac{1}{5}

The  \displaystyle y-intercept is \displaystyle \left (0, - \frac{1}{5} \right ).

 

Example Question #8 : How To Find X Or Y Intercept

Give the \displaystyle x-intercept of the line that passes through points \displaystyle (-3, 4) and \displaystyle (2, -3).

Possible Answers:

\displaystyle \left (- \frac{1}{7} , 0 \right )

\displaystyle \left ( - \frac{1}{5},0 \right )

\displaystyle \left ( - 8 \frac{1}{5},0 \right )

\displaystyle \left (- 5\frac{6}{7} , 0 \right )

\displaystyle \text{The line has no } x\text{-intercept.}

Correct answer:

\displaystyle \left (- \frac{1}{7} , 0 \right )

Explanation:

First, find the slope of the line, using the slope formula

\displaystyle m = \frac{y_{2} -y_{1}}{x_{2}-x_{1}}

setting \displaystyle x_{1} = -3, y_{1} = 4, x_{2} = 2, y_{2} = -3:

\displaystyle m = \frac{-3 -4}{2-(-3)} = \frac{-7}{5} =- \frac{7}{5}

By the point-slope formula, this line has the equation

\displaystyle y - y_{1} = m(x -x_{1})

where

\displaystyle x_{1} = -3, y_{1} = 4, m = - \frac{7}{5}; the line becomes

\displaystyle y - 4 = - \frac{7}{5}(x -(-3))

or

\displaystyle y - 4 = - \frac{7}{5}(x +3)

To find the \displaystyle x-intercept, substitute 0 for \displaystyle y and solve for \displaystyle x:

\displaystyle 0 - 4 = - \frac{7}{5}(x +3)

\displaystyle - 4 = - \frac{7}{5} x - \frac{21}{5}

\displaystyle - 4+ \frac{21}{5} = - \frac{7}{5} x - \frac{21}{5}+ \frac{21}{5}

\displaystyle \frac{1}{5} = - \frac{7}{5} x

\displaystyle - \frac{5}{7} \cdot \frac{1}{5} = - \frac{5}{7}\left ( - \frac{7}{5} x \right )

\displaystyle - \frac{1}{7} =x

The  \displaystyle x-intercept is \displaystyle \left (- \frac{1}{7} , 0 \right ).

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