Algebra 1 : Slope and Line Equations

Study concepts, example questions & explanations for Algebra 1

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Example Questions

Example Question #1 : Slope And Line Equations

Given two points, (5, –8) (–2, 6), what is the equation of the line containing them both?

Possible Answers:

y = 2x – 2

y = (2/7)x – 8

y = –2x + 2

No Solution

y = (–2/7)x + 8

Correct answer:

y = –2x + 2

Explanation:

First, you should plug the given points, (5, –8) (–2, 6), into the slope formula to find the slope of the line. 

\displaystyle slope=\frac{y_2-y_1}{x_2-x_1}

\displaystyle slope=\frac{-8-6}{5-(-2)}=\frac{-8-6}{5+2}=\frac{-14}{7}=-2

Then, plug the slope into the slope formula, y = mx + b, where m is the slope.

y = –2x + b

Plug in either one of the given points, (5, –8) or (–2, 6), into the equation to find the y-intercept (b). 

6 = –2(–2) + b

6 = 4 + b

2 = b

Plug in both the slope and the y-intercept into slope intercept form. 

y = –2x + 2

Example Question #1 : Slope And Line Equations

What is the equation of a line with slope of 3 and a y-intercept of –5? 

Possible Answers:

y = 3x – 5

y = (3/5)x + 2

y = 3x + 5

y = 5x – 3

y = –5x + 3

Correct answer:

y = 3x – 5

Explanation:

These lines are written in the form y = mx + b, where m is the slope and b is the y-intercept. We know from the question that our slope is 3 and our y-intercept is –5, so plugging these values in we get the equation of our line to be y = 3x – 5.

m = 3 and b = –5

Example Question #3 : How To Find The Equation Of A Line

A line contains the points (8, 3) and (-4, 9). What is the equation of the line?

Possible Answers:

\displaystyle y=--2x+7

\displaystyle y=\frac{1}{2}x+5

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

\displaystyle y=-2x+1

\displaystyle y=2x+7

Correct answer:

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

Explanation:

\displaystyle y=mx+b is the slope-intercept form of the equation of a line.

Slope \displaystyle m is equal to \displaystyle \frac{rise}{run} between points, or \displaystyle \frac{6}{-12}.

So \displaystyle m=-\frac{1}{2}.

At point (8, 3 ) the equation becomes

\displaystyle 3=-\frac{1}{2}\cdot 8+b\rightarrow 3=-4+b

So \displaystyle b=7

Example Question #2 : How To Find The Equation Of A Line

Given two points \displaystyle A \displaystyle \left ( 1,5 \right ) and \displaystyle B\displaystyle \left ( 0,2 \right ), find the equation of a line that passes through the point \displaystyle \left ( -3,-5 \right ) and is parallel to the line passing through points \displaystyle A and \displaystyle B.

Possible Answers:

\displaystyle x-3y=-12

\displaystyle 3x-y=-4

\displaystyle 3x-y=4

\displaystyle 2x+2y=6

\displaystyle x-3y=-4

Correct answer:

\displaystyle 3x-y=-4

Explanation:

The slope of the line passing through points \displaystyle A and \displaystyle B can be computed as follows:

\displaystyle Slope = \frac{y_{2}-y_{1}}{x_{2}-x_{1}}=\frac{5-2}{1-0}=3

Now, the new line, since it is parallel, will have the same slope.  To find the equation of this new line, we use point-slope form:

\displaystyle y-y_{1}=m(x-x_{1}), where \displaystyle m is the slope and \displaystyle \left ( x_{1},y_{1} \right ) is the point the line passes through.

\displaystyle y-(-5)=3(x-(-3))

\displaystyle y+5-3(x+3)

\displaystyle y+5=3x+9

After rearranging, this becomes \displaystyle 3x-y=-4

Example Question #3621 : Algebra 1

Find the equation, in \displaystyle y=mx+b form, of the line that contains the points \displaystyle (2,10) and \displaystyle (-4,13).

Possible Answers:

\displaystyle y=-\frac{1}{2}x+11

\displaystyle y=-2x+5

\displaystyle y=11x-5

\displaystyle y=2x+6

\displaystyle y=\frac{1}{2}x+9

Correct answer:

\displaystyle y=-\frac{1}{2}x+11

Explanation:

When finding the equation of a line from some of its points, it's easiest to first find the line's slope, or \displaystyle m.

To find slope, divide the difference in \displaystyle y values by the difference in \displaystyle x values. This gives us \displaystyle 3 divided by \displaystyle -6, or \displaystyle -\frac{1}{2}.

Next, we just need to find \displaystyle b, which is the line's \displaystyle y-intercept. By plugging one of the points into the equation \displaystyle y = -\frac{1}{2}x+b, we obtain a \displaystyle b value of 11 and a final equation of

\displaystyle y=-\frac{1}{2}x+11

Example Question #3621 : Algebra 1

What is the equation of a straight line that connects the points indicated in the table?

Question_5

Possible Answers:

\displaystyle y=2x+6

\displaystyle y=4x+3

\displaystyle y=x+6

\displaystyle y=3x+4

Correct answer:

\displaystyle y=3x+4

Explanation:

We can find the equation of th line in slope-intercept form by finding \displaystyle m and \displaystyle b.

First, calculate the slope, \displaystyle m, for any two points. We will use the first two.

\displaystyle m=\frac{y_{2}-y_1}{x_2-x_1}=\frac{7-4}{1-0}=\frac{3}{1}=3

Next, using the slope and any point on the line, calculate the y-intercept, \displaystyle b. We will use the first point.

\displaystyle y=mx+b

\displaystyle 4=3(0)+b

\displaystyle 4=b

The correct equation in slope-intercept form is \displaystyle y=3x+4.

Example Question #3 : Slope And Line Equations

What is the equation of a line with a slope of \displaystyle 2 and a \displaystyle y-intercept of \displaystyle -12?

Possible Answers:

\displaystyle y=-12x+2

\displaystyle y=2x+12

\displaystyle y=2x-12

None of the above

\displaystyle y=x+6

Correct answer:

\displaystyle y=2x-12

Explanation:

When a line is in the \displaystyle y=mx+b format, the \displaystyle m is its slope and the \displaystyle b is its \displaystyle y-intercept. In this case, the equation with a slope of \displaystyle 2 and a \displaystyle y-intercept of \displaystyle -12 is \displaystyle y=2x-12.

Example Question #8 : How To Find The Equation Of A Line

In 1990, the value of a share of stock in General Vortex was $27.17. In 2000, the value was $48.93. If the value of the stock rose at a generally linear rate between those two years, which of the following equations most closely models the price of the stock, \displaystyle p, as a function of the year, \displaystyle x?

Possible Answers:

\displaystyle p(x)=-2.176x+4,400.93

\displaystyle p(x)=2.176x-4,400.93

\displaystyle p(x)=2.176x-4,303.07

\displaystyle p(x)=2.176x-2,106.47

\displaystyle p(x)=-2.176x+4,303.07

Correct answer:

\displaystyle p(x)=2.176x-4,303.07

Explanation:

We can treat the price of the stock as the \displaystyle y value and the year as the \displaystyle x value, making any points take the form \displaystyle (year, price), or \displaystyle (x,p). This question is asking for the line that includes points \displaystyle (1990,27.17) and \displaystyle (2000,48.93)

To find the equation, first, we need the slope.

\displaystyle m= \frac{48.93-27.17}{2000-1990}= \frac{21.76}{10}=2.176

Now use the point-slope formula with this slope and either point (we will choose the second).

\displaystyle y-48.93 = 2.176(x-2000)

\displaystyle y-48.93 = 2.176x-4352

\displaystyle y = 2.176x-4303.07

Example Question #4 : How To Find The Equation Of A Line

Possible Answers:

\displaystyle y=\frac{-x}{2}-1

\displaystyle y=-2x+1

\displaystyle y=\frac{x}{2}-1

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

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

Correct answer:

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

Explanation:

\displaystyle 2b-4 = -2\;\;\;\;\;\;2b = 2\;\;\;\;\;b=1

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

Example Question #2 : Slope And Line Equations

Which of these lines has a slope of 5 and a \displaystyle y-intercept of 6?

Possible Answers:

\displaystyle y=6x+5

\displaystyle y=-5x-6

 

\displaystyle y=x+5

\displaystyle y=\frac{1}{5}x+6

\displaystyle y=5x+6

Correct answer:

\displaystyle y=5x+6

Explanation:

When an equation is in the \displaystyle y=mx+b form, the \displaystyle m indicates its slope while the \displaystyle b indicates its \displaystyle y-intercept. In this case, we are looking for a line with a \displaystyle m of 5 and a \displaystyle b of 6, or \displaystyle y=5x+6.

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