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**LINEAR EQUATIONS MSJC ~ Menifee Valley Campus**

Math Center Workshop Series Janice Levasseur

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Equations of the Line Write the equation of a line given the slope and the y-intercept Write the equation of a line given the slope and a point Write the equation of a line given two points

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Equations of the Line Write the equation of a line given the slope and the y-intercept: m and (0, b) Write the equation of a line given the slope and a point Write the equation of a line given two points

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**Ex: Find an equation of the line with slope = 6 and y-int = (0, -3/2)**

Recall: slope-intercept form of a linear equation y = mx + b, where m and b are constants Given the y-int = (0, -3/2) b = - 3/2 Given the slope = 6 m = 6 Putting everything together we get the equation of the line in slope-int form: y m 6 x + - 3/2 b = y = 6x – 3/2

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**Ex: Find an equation of the line with slope = 1. 23 and y-int = (0, 0**

Recall: slope-intercept form of a linear equation y = mx + b, where m and b are constants Given the y-int = (0, 0.63) b = 0.63 Given the slope = 1.23 m = 1.23 Putting everything together we get the equation of the line in slope-int form: y m + b = 1.23 x 0.63 y = 1.23x

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Equations of the Line Write the equation of a line given the slope and the y-intercept Write the equation of a line given the slope and a point: m and (x1, y1) Write the equation of a line given two points

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**Ex: Find an equation of the line with slope = -3 that contains the point (4, 2)**

Start with the slope-intercept form of a linear equation y = mx + b Slope = - 3 y = - 3x + b What is b, though? What is b, though? Use the given point (4, 2) x = 4 and y = 2 y = - 3x + b 2 = - 3(4) + b put it together 2 = b we have m and b 14 = b y = - 3 x + 14

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**Ex: Find an equation of the line with slope = -0**

Ex: Find an equation of the line with slope = that contains the point (2, -6) Start with the slope-intercept form of a linear equation y = mx + b Slope = y = x + b What is b, though? Use the given point (2, -6) x = 2 and y = -6 y = x + b -6 = -0.25(2) + b put it together -6 = b we have m and b -5.5 = b y = -0.25x – 5.5

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Equations of the Line Write the equation of a line given the slope and the y-intercept Write the equation of a line given the slope and a point Write the equation of a line given two points: (x1, y1) and (x2, y2)

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**Ex: Find an equation of the line containing the points (-2, 1) and (3, 5)**

First, find the slope of the line containing the points: Slope = m = rise = y1 - y2 = 1 – (5) run x1 - x – 3 Now we have m = 4/5 and two points. Pick one point and proceed like in the last section.

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**We have m = 4/5, the point (-2, 1), and y = mx + b**

Slope = 4/5 y = 4/5x + b What is b, though? Use the given point (-2, 1) x = -2 and y = 1 y = 4/5x + b 1 = 4/5(-2) + b 1 = (-8/5) + b 13/5 = b put it together we have m and b y = 4/5x + 13/5

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**Ex: Find an equation of the line containing the points (-4, 5) and (-2, -3)**

First, find the slope of the line containing the points: Slope = m = rise = y1 - y2 = 5 – (-3) run x1 - x – (-2) = -4 Now we have m = -4 and two points. Pick one point and proceed like in the last section.

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**We have m = -4, the point (-4, 5), and y = mx + b**

Slope = -4 y = -4x + b What is b, though? Use the given point (-4, 5) x = -4 and y = 5 y = -4x + b 5 = -4(-4) + b 5 = 16 + b -11 = b put it together we have m and b y = -4x – 11

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**Ex: Find an equation of the line containing the points (0, 0) and (1, -5)**

First, find the slope of the line containing the points: Slope = m = rise = y1 - y2 = 0 – (-5) run x1 - x – (1) = -5 Now we have m = -5 and two points. Pick one point and proceed like in the last section.

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**We have m = -5, the point (0, 0), and y = mx + b**

Slope = -5 y = -5x + b What is b, though? Use the given point (0, 0) x = 0 and y = 0 y = -5x + b 0 = -5(0) + b 0 = 0 + b 0 = b put it together we have m and b y = -5x + 0 y = -5x

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Equations of the Line Write the equation of a line given the slope and the y-intercept Write the equation of a line given the slope and a point Write the equation of a line given two points

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**Parallel & Perpendicular Lines**

When we graph a pair of linear equations, there are three possibilities: the graphs intersect at exactly one point the graphs do not intersect the graphs intersect at infinitely many points We will consider a special case of situation 1 and also situation 2.

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**Perpendicular Lines (Situation 1)**

Perpendicular lines intersect at a right angle Notation: L1: y = m1x + b1 L2: y = m2x + b2 L1 ^ L2

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**Nonvertical perpendicular lines have slopes that are the negative reciprocals of each other:**

m1m2 = -1 ~ or ~ m1 = - 1/m2 ~ or ~ m2 = - 1/m1 If l1 is vertical (l1: x = a) and is perpendicular to l2, then l2 is horizontal (l2: y = b) ~ and ~ vice versa

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Ex: Determine whether or not the graphs of the equations of the lines are perpendicular: l1: x + y = 8 and l2: x – y = - 1 First, determine the slopes of each line by rewriting the equations in slope-intercept form: l1: y = - x + 8 and l2: y = x + 1 1 1 m1 = and m2 = -1 1 Since m1m2 = (-1)(1) = -1, the lines are perpendicular.

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Ex: Determine whether or not the graphs of the equations of the lines are perpendicular: l1: -2x + 3y = -21 and l2: 2y – 3x = 16 First, determine the slopes of each line by rewriting the equations in slope-intercept form: l1: y = (2/3)x - 7 and l2: y = (3/2)x + 8 m1 = and m2 = 2/3 3/2 Since m1m2 = (2/3)(3/2) = 1 = -1 Therefore, the lines are not perpendicular!

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**Parallel Lines (Situation 2)**

Parallel lines do not intersect Notation: L1: y = m1x + b1 L2: y = m2x + b2 L1 || L2

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**Nonvertical parallel lines have the same slopes but different y-intercepts:**

m1 = m2 ~ and ~ b1 = b2 Horizontal Parallel Lines have equations y = p and y = q where p and q differ. Vertical Parallel Lines have equations x = p and x = q where p and q differ.

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Ex: Determine whether or not the graphs of the equations of the lines are parallel: l1: 3x - y = -5 and l2: y – 3x = - 2 First, determine the slopes and intercepts of each line by rewriting the equations in slope-intercept form: l1: y = 3x + 5 and l2: y = 3x - 2 m1 = and m2 = 3 3 b1 = and b2 = 5 -2 Since m1 = m2 and b1 = b2 the lines are parallel.

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Ex: Determine whether or not the graphs of the equations of the lines are parallel: l1: 4x + y = 3 and l2: x + 4y = - 4 First, determine the slopes and intercepts of each line by rewriting the equations in slope-intercept form: l1: y = -4x + 3 and l2: y = (-¼)x - 1 m1 = and m2 = -4 - ¼ Since m1 = m2 the lines are not parallel.

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EXAMPLE 1 Write an equation of a line from a graph

EXAMPLE 1 Write an equation of a line from a graph

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