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Let's graph these lines:

y = 2x and y = -( 1 / 2 )x

* I'm using y = mx + b stuff to graph them!
 

a graph of the lines y = 2x and y = -( 1 / 2 )x ... they are perpendicular lines These lines are perpendicular.

(They form a 90 degreeangle.)

So, what's going on with the slopes?

m1 = 2 and m2 = -1 / 2
Notice that they are inverses
but the signs are different

A typical math book would say:

m1 times m2 = ( 2 ) ( -1 / 2 ) = -1

But this is easier:

You flip it over and change the sign --
That's how you get the slopes of perpendicular lines.

* If you want to be a super geek, you can rap it!