



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 
