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Elementary Algebra Written Homework
Chapter 3 Part B
Creating equations of lines [3.6 & 3.7]
For most standard lines, one of these two formulas is used to create an equation. It is a common mistake for
students to attempt to always use point-slope form, perhaps because it is a simpler formula. However, it is
necessary to choose the formula that is suited to the information that you are given instead.
Slope-intercept form [3.6]
Point-slope form [3.7]
y = mx + b
y ! y1 = m ( x = x1 )
Use if you know slope m & y-intercept ( 0, b )
Use if you know slope m & any one point ( x1, y1 )
Example of a common mistake Find the slope-intercept equation of the line with slope
y = mx + b
2
, passing through the point ( !3, !5) .
3
! !
2
( "5)
3
2
y = x!5
3
This is incorrect, because !5 is the y-coordinate of a point on the line, but that point is not the y-intercept. We
know this because the x-coordinate is not equal to 0. Instead, it is necessary to use the point-slope form.
1. Find the equation described in the above problem, correctly.
2. Find the slope-intercept equation of the line with slope !4 and y-intercept ( 0, !7) .
a. What is the more appropriate form to use to find this equation?
b. Will either form work?
Why or why not?
c. Find the equation.
Important note Horizontal and vertical lines are handled differently. Don’t try to fit them to either of these forms.
Ch 3B
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Graphing using slope [3.6 & 3.7]
Make sure you can draw accurate graphs with a pencil, without the assistance of the computer graphing tools.
3. Graph. y = !
3
x!2
2
4. Graph the line with slope
Ch 3B
2
that passes through the point ( 3, 4 ) .
5
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