Tuesday, February 16, 2016

another thing not to do...

Suppose you are given two planes, say:
5x-2y+z-3=0  and -x+y+2z-1=0
and you want the intersection of these two planes, which is a line, and 0=0 so you should set these equations equal each other
5x-2y+z-3=-x+y+2z-1, 
which gives you 
6x-3y-z-2=0,
right?  

DO NOT DO THIS.

Do not do this because your are trying to find the points <x,y,z> that make both equations true. Yeah, both equations are equal to zero and so 5x-2y+z-3=-x+y+2z-1 when x,y,z satisfy both equations, but 5x-2y+z-3=-x+y+2z-1 for a lot of triples  <x,y,z> that don't make either equation true.  You need to figure out how to solve both equations at once and this won't help you do this.

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