Question

Two (or more) linear functions are perpendicular if

Answer 1

Simplified, their slopes multiply to -1. 

What that means is that if there's a equation 
$y- y_{1} =m(x- x_{1} )$ , if a line is perpendicular , that line's m value multiplied by this line's m value equals -1. For example, if m=1/4 for one value, the first thing we do is make it negative to get -1/4. Next, we switch the numerator and denominator to get -4. If there is a number like 5, the fraction of it is 5/1 (since 5 divided by 1 is 5) and the reciprocal (or the numerator and denominator switched) is 1/5.  This also applied to y=mx+b. 

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