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How to use Algebra to find parallel and perpendicular lines. Parallel LinesHow do we know when two lines are parallel?
Their slopes are the same!
Example:Find the equation of the line that is:
The slope of y=2x+1 is: 2 The parallel line needs to have the same slope of 2. We can solve it using the "point-slope" equation of a line: y − y1 = 2(x − x1) And then put in the point (5,4): y − 4 = 2(x − 5) And that answer is OK, but let's also put it in y = mx + b form: y − 4 = 2x − 10 y = 2x − 6 Vertical LinesBut this does not work for vertical lines ... I explain why at the end. Not The Same LineBe careful! They may be the same line (but with a different equation), and so are not parallel. How do we know if they are really the same line? Check their y-intercepts (where they cross the y-axis) as well as their slope:
For y = 3x + 2: the slope is 3, and y-intercept is 2 For y − 2 = 3x: the slope is 3, and y-intercept is 2 In fact they are the same line and so are not parallel Perpendicular LinesTwo lines are Perpendicular when they meet at a right angle (90°). To find a perpendicular slope:
When one line has a slope of m, a perpendicular line has a slope of −1m In other words the negative reciprocal
Example:Find the equation of the line that is
The slope of y=−4x+10 is: −4 The negative reciprocal of that slope is: m = −1−4 = 14 So the perpendicular line will have a slope of 1/4: y − y1 = (1/4)(x − x1) And now put in the point (7,2): y − 2 = (1/4)(x − 7) And that answer is OK, but let's also put it in "y=mx+b" form: y − 2 = x/4 − 7/4 y = x/4 + 1/4 Quick Check of PerpendicularWhen we multiply a slope m by its perpendicular slope −1m we get simply −1. So to quickly check if two lines are perpendicular:
When we multiply their slopes, we get −1 Like this:
Are these two lines perpendicular?
When we multiply the two slopes we get: 2 × (−0.5) = −1 Yes, we got −1, so they are perpendicular. Vertical LinesThe previous methods work nicely except for a vertical line: In this case the gradient is undefined (as we cannot divide by 0): m = yA − yBxA − xB = 4 − 12 − 2 = 30 = undefined So just rely on the fact that:
Summary
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The calculator will find the equation of the parallel/perpendicular line to the given line passing through the given point, with steps shown. For drawing lines, use the graphing calculator.
SolutionYour input: find the equation of the line parallel to the line $$$y=2 x + 5$$$ passing through the point $$$\left(-3,5\right)$$$. The equation of the line in the slope-intercept form is $$$y=2 x + 5$$$. The slope of the parallel line is the same: $$$m=2$$$. So, the equation of the parallel line is $$$y=2 x+a$$$. To find $$$a$$$, we use the fact that the line should pass through the given point: $$$5=\left(2\right) \cdot \left(-3\right)+a$$$. Thus, $$$a=11$$$. Therefore, the equation of the line is $$$y=2 x + 11$$$. Answer: $$$y=2 x + 11$$$.
Parallel lines are coplanar lines that do not intersect. In two dimensions, parallel lines have the same slope .
We can write the equation of a line parallel to a given line if we know a point on the line and an equation of the given line.
Example: Write the equation of a line that passes through the point ( 3 , 1 ) and is parallel to the line y = 2 x + 3 . Parallel lines have the same slope. The slope of the line with equation y = 2 x + 3 is 2 . So, any line parallel to y = 2 x + 3 has the same slope 2 . Now use the point-slope form to find the equation. y − y 1 = m ( x − x 1 ) We have to find the equation of the line which has slope 2 and passes through the point ( 3 , 1 ) . So, replace m with 2 , x 1 with 3 , and y 1 with 1 . y − 1 = 2 ( x − 3 ) Use the distributive property . y − 1 = 2 x − 6 Add 1 to each side. y − 1 + 1 = 2 x − 6 + 1 y = 2 x − 5 Therefore, the line y = 2 x − 5 is parallel to the line y = 2 x + 3 and passes through the point ( 3 , 1 ) .
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