Algebra 1 A Test For Perpendicular Equations
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Play The Angles
A Test For Perpendicular Equations

 
 If two lines cross and make 90 degree angles between them
 they are called "perpendicular" to each other ...
 

 
 There is an easy test to see if two equation lines
 form 90 degree angles where they cross.
 If they do, then ...
 

(slope of one line) x (slope of the other line) = -1

 
 Example:
 
 Find out if the graph lines of the equations ...
 

3X - 5Y = 7 and 6Y = -10X + 5

 
 are perpendicular.
 
 An easy way to find the slope of an equation
 is to make the equation look like ...
 

Y = (some number)X + (some other number)

 
 So if you have an equation like  ...
 

3X - 5Y = 7

 
 move it around until it looks like Y = (stuff).
 

 

 So the slope of this equation line is 3/5.
 The line goes up from left to right.
 
 Now let's deal with 6Y = -10X + 5 ...
 

 
 So the slope of this equation line is -10/6.
 That simplifies to -5/3.
 
 Now we can multiply the two slopes together,
 and see what we get ...
 

 
 When we multiply these slopes together we get -1.
 That means the two equation lines are perpendicular.
 
 Thrilling, isn't it ...
 

   copyright 2005 Bruce Kirkpatrick

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