Understanding Non-Function Polar Equations in Calculus II

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Homework Help Overview

The discussion revolves around polar equations in Calculus II, specifically focusing on identifying polar equations that do not represent functions. Participants explore the differences between polar and Cartesian representations and the implications of the vertical line test in this context.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the nature of polar equations that yield multiple outputs for a single input, questioning how certain equations behave differently in polar versus Cartesian coordinates. They consider examples like r=1 and r^2=theta^2, while also reflecting on the implications of equations like x^2=y^2.

Discussion Status

The discussion is active, with participants sharing examples and clarifying their understanding of polar equations. Some guidance has been offered regarding potential examples, and there is a sense of increased clarity among participants about the topic.

Contextual Notes

Participants are navigating the complexities of polar equations and their relationship to Cartesian functions, with some expressing confusion about the definitions and implications of these concepts.

lovelylila
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We're currently working on polar equations in Calculus II. I'm very confused about one point of our discussion today, finding a polar equation that is not a function (ie, one value plugged in will give you two output values). My teacher mentioned how you can't use the vertical line test for polar equations, and that some equations that are not functions in the Cartesian plane are functions in the polar plane? I'd appreciate any help or guidance regarding an example of a polar equation that is not a function because I'm completely bewildered!
 
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I assume you want a polar function that's not a function in the x-y plane? Try r=1. Can't get much simpler than that.
 
no, I'm looking for a polar equation that is not a function in the polar plane.
 
Well, x^2=y^2 is not a function in the cartesian plane. Why not? Does that help you think of a polar equation that would have similar problems?
 
its not a function because say if you plug 2 in for x, y can be either -2 or +2. i understand that. well, in the polar plane that equation would be (rcostheta)^2= (rsintheta)^2. i guess that works, since the rs would cancel out leaving you with (costheta)^2= (sintheta)^2 but it seems too simple. is this right? i really appreciate your help! :-)
 
I was thinking of r^2=theta^2, but that works, too. If you know r, that doesn't determine theta since theta can be pi/4, 3pi/4, 5pi/4... And even you know theta is one of those values, it doesn't determine r. So, no. Definitely not a function. It is simple.
 
thank you very much! i have a much clearer idea of what is going on now! :-) a thousand thanks!
 
lovelylila said:
We're currently working on polar equations in Calculus II. I'm very confused about one point of our discussion today, finding a polar equation that is not a function (ie, one value plugged in will give you two output values). My teacher mentioned how you can't use the vertical line test for polar equations, and that some equations that are not functions in the Cartesian plane are functions in the polar plane? I'd appreciate any help or guidance regarding an example of a polar equation that is not a function because I'm completely bewildered!

Are you trying to find a curve such that r is a function of \theta but y is not a function of x? If so, r= 1, as Dick said, works. If you are looking for some equation in which r is not a function of \theta, r^2= \theta^2 is an obvious choice.
 

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