Proving Orthogonal Curves at Intersection Point

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Discussion Overview

The discussion focuses on proving that two curves are orthogonal at their intersection point. Participants explore the concept of orthogonality in the context of calculus, specifically regarding the slopes of tangent lines to the curves at the point of intersection.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant inquires about the method to prove orthogonality, suggesting that comparing the slopes of the derivatives is necessary and that they should be negative reciprocals.
  • Another participant questions what would convince the first participant that their ideas are correct.
  • A participant expresses a desire to find two equations that are orthogonal, indicating difficulty in locating examples in their Calculus book.
  • There is a clarification on the definition of orthogonal curves, with a participant stating that for smooth curves, the tangent lines must be orthogonal.
  • One participant asks for specific equations to work with, indicating a need for practical examples.
  • A suggestion is made to develop the basic idea into an equation, implying that the foundational understanding is present.
  • Examples of orthogonal curves are provided, such as y = x being orthogonal to y = -x, along with a challenge to find the equation of a line orthogonal to y = x² at a specific point.
  • A more complex example is discussed regarding the family of curves orthogonal to a given family, detailing the differentiation process and resulting equations.

Areas of Agreement / Disagreement

Participants generally agree on the basic concept of orthogonality involving the slopes of tangent lines, but there is no consensus on specific examples or methods for proving orthogonality, as some seek further clarification and examples.

Contextual Notes

Some participants express uncertainty about finding appropriate equations and examples, indicating a reliance on definitions and the need for additional context in their understanding of orthogonal curves.

gokugreene
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How do I prove that two curves are orthogonal when they interest each other at a specific point?

Do I just take the derivative of both and compare the slopes?
The slopes should be negative reciprocals of each other, correct?
 
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well since you have good ideas, what would convince you they are right?
 
I believe I am correct; however, I do wish to prove it.
I need two equations that are orthogonal.
I can't find any in my Calculus book.

Thanks
 
do you mean you want the definition of orthogonal curves? to me it would be as you assume, i.e. at least for smooth curves, that their tangent lines are orthogonal.
 
Yea that works, but do you have any equations I could play with?

Thanks :)
 
You've got the basic idea, so what's stopping you from developing that idea into an equation?
 
Do you mean you want examples?


y= x is orthogonal to y= -x!

Can you find the equation of the line orthogonal to y= x2 at (1,1)?

(Find the derivative of x2 at x= 1. Yes, the slope of a line orthogonal to that is the negative of the reciprocal of the derivative.)

A common problem in differential equations is to find the family of curves orthogonal to a given family: To find the set of all curves orthogonal to the family of curves
xy= a, differentiate with respect to x: y+ xy'= 0 (thus eliminating the constant a).
Then y'= -y/x at each point (except x= 0). Any curve orthogonal to that must have
y'= x/y or yy'= x so (1/2)y2= (1/2)x2+ C or
x2- y2= c (c= -2C). The family of curves orthogonal to the hyperbolas having the axes as asymptotes is the family of hyperbolas having y= x and y= -x as asymptotes.
 

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