2.5.2 – Writing Equations of Lines
Cont’d
• Yesterday, we could write equations of lines in a few ways
• 1) Given slope and y-intercept (y = mx + b)
• 2) Given a graph of a line, determine slope and y-intercept
(also write in y = mx + b)
• 3) Use point-slope form given any point and slope of a line
(y - y1 = m(x - x1))
– Could write in y = mx + b as well if we simplified it
• We’ll start off still using point-slope form, but
this time in a different scenario
• Remember, only need 2 points to represent a
line
• Now, you will be given two points, and you
will need to find an equation of that line
2 points
• Using two points, we will;
• 1) Find the slope, using the slope formula;
y2  y1
m = x x
2
1
• 2) Using any of the two points, write in pointslope form
• 3) Simplify as y = mx + b
• Example. Write an equation of the line that
passes through the points (-2, 3) and (2, -5).
• Slope = m =
• Point-Slope form;
• Slope-Intercept form;
• Example. Write an equation of the line that
passes through the points (-8, 10) and (-2, 17).
• Slope = m =
• Point-Slope form;
• Slope-Intercept form;
Graphically
• Similar to yesterday, you can determine the
equation of a line graphically, even if we
cannot accurately locate the y-intercept
• Just find two known points, then
• 1) Find the slope using rise/run (counting)
• 2) Pick one point, use point-slope formula
• Example. Write an equation of the line shown.
• Example. Write an equation of the line shown.
Parallel/Perpendicular Equations
• Recall, we covered parallel and perpendicular
lines
• What determines if two lines are parallel?
• What determines if two lines are perpendicular?
• Using given info, we will once again use pointslope form after finding each slope
• Example. Write an equation of the line that is
parallel to y = 4x – 5 and goes through the
point (-1, 3).
• Example. Write an equation of the line that is
perpendicular to y = 4x – 5 and goes through
the point (-1, 3).
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Assignment
Pg. 98
24, 34-52 even, 59, 60
Pg. 100
11-13