Please Help With A Proof

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Please Help With A Proof!!

If L1 is perpendicular to L2, prove that the slope of L2 is the negative reciprocal of the slope of L1..


PLEASE HELP ME!!
 

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  • #2
EnumaElish
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What is the definition of perpendicular? definition of slope? Any ideas?
 
  • #3
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What is the definition of perpendicular? definition of slope? Any ideas?

Perpendicular is when two lines intersect to form 90 degree angles, slope is whether the line is moving up or down, and how much like the top value is the y and the bottom is the x.
 
  • #4
rock.freak667
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If I tell you that the tangent of the angle that a line makes with any horizontal to it gives you the gradient. does that help?
 
  • #5
Dick
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Draw the right triangle connecting the points (0,0), (x,0) and (x,y). Got it? So the hypotenuse has slope y/x. Now rotate it 90 degrees and tell me what the endpoint of that hypotenuse is? What's the slope of that hypotenuse?
 
  • #6
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My professor doesn't want us to use trigonometry cuz we didn't learn that yet..and Dick, im not sure im allowed to use that as a proof..
 
  • #7
Dick
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My professor doesn't want us to use trigonometry cuz we didn't learn that yet..and Dick, im not sure im allowed to use that as a proof..

If you can't rotate a point 90 degrees, then what's your definition of perpendicular?
 
  • #8
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If you can't rotate a point 90 degrees, then what's your definition of perpendicular?

During class all he told us was to prove that if
1). L1 is perpendicular to L2
2). slope of L1 is M1 where M1 can't be 0

prove that slope of L2 is -1/M1...thats it and he said not to use anything we didn't learn, he didnt talk about rotating anything, or trignometry. =(
 
  • #9
Dick
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If you don't know that the line through the point (x,y) and (0,0) and the line through the point (-y,x) and (0,0) are perpendicular then I don't think we are going anywhere. What IS your definition of perpendicular?
 
  • #10
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Well i can't use my definition all he said was his definition, and thats that they intersect and the slope of l2 is negative recipricol and thats it...we have to prove that
 
  • #11
Dick
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Well i can't use my definition all he said was his definition, and thats that they intersect and the slope of l2 is negative recipricol and thats it...we have to prove that

Oh, come on. You said the definition of perpendicular is that the lines meet at 90 degree angles. This means you can use some geometry. Use it.
 
  • #12
EnumaElish
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Did you learn to define perpendicular in terms of inner products (of two vectors)?
 
  • #13
HallsofIvy
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Well i can't use my definition all he said was his definition, and thats that they intersect and the slope of l2 is negative recipricol and thats it...we have to prove that
That makes no sense. Apparently you want to PROVE that his definition is... what? Equivalent to some other definition, apparently! You can't PROVE that his definition is correct using only his definition! What is YOUR definition of perpendicular? That two lines intersect at 90 degrees?
 
  • #14
In graph (orthonormal system) we know that if two lines are perpendicular then slope 1*slope 2 = -1 (it is a rule)
so slope 1 = -1/slope 2
 
  • #15
EnumaElish
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In graph (orthonormal system) we know that if two lines are perpendicular then slope 1*slope 2 = -1 (it is a rule)
so slope 1 = -1/slope 2
One man's definition can be another's problem. :smile:
 
  • #16
What is the lesson you took ? to determine the proof ?
 

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