# Domain and range, functions of 2 variables

by hotcommodity
Tags: domain, functions, range, variables
 P: 436 I'm having a bit of trouble grasping the domain and range of functions of 2 variables. Does anyone know of any helpful tutorials that will help me get the hang of this concept? Any help appreciated.
 P: 1,705 tutorial starts here and now $$f(x,y)=\frac{1}{\sqrt{x^2+y^2}}$$ what is the domain and range of this function?
 P: 436 Its domain is x^2 + y^2 must be greater than or equal to zero, and its range is from minus to plus infinity?...
P: 1,705
Domain and range, functions of 2 variables

 Quote by hotcommodity Its domain is x^2 + y^2 must be greater than or equal to zero, and its range is from minus to plus infinity?...
do you know what domain means? you're like 1/3 of the way there. part of your answer is wrong and you didn't answer the question about the domain. but you do have something...

the range is actually discontinuous but again you're close

edit

the domain is actually discontinuous

can the function ever be negative? did you just guess?
P: 1,075
 Quote by ice109 the range is actually discontinuous but again you're close
How is the range of that function discontinuous?
P: 1,705
 Quote by d_leet How is the range of that function discontinuous?
bah i meant domain, and i just realized the grievous error he made in the range as well.
 P: 1,705 you've probably confused the hell out of him
 P: 436 ice's post kind of inspired me to take the bull by the horns, so I opened up a calc book of mine, different from my class text, and I think I've got a handle on it. The domain would be D = {(x,y)|x^2 + y^2 >0}, and the range would be R = { z | z = f(x,y), x^2 + y^2 >0}. Is that right?
HW Helper
P: 3,682
 Quote by hotcommodity ice's post kind of inspired me to take the bull by the horns, so I opened up a calc book of mine, different from my class text, and I think I've got a handle on it. The domain would be D = {(x,y)|x^2 + y^2 >0}, and the range would be R = { z | z = f(x,y), x^2 + y^2 >0}. Is that right?
Yes, but you should probably simplify those. Are you working over the reals or the complex numbers? Oh, of course you are working over the reals -- you used a >.

So for which (x, y) in R^2 is it true that x^2 + y^2 > 0? The strict quadrants, right... $x\neq0\neq y$. What is the range simplified the same way?
P: 436
 Quote by CRGreathouse Yes, but you should probably simplify those. Are you working over the reals or the complex numbers? Oh, of course you are working over the reals -- you used a >. So for which (x, y) in R^2 is it true that x^2 + y^2 > 0? The strict quadrants, right... $x\neq0\neq y$. What is the range simplified the same way?
I'm not quite sure. I know the range is in R^1, and the range is every value that f(x, y) can take on... What are the "strict quadrants?"

Edit: Also, since the function takes an pair (x, y) from R^2, and turns it into R^1, is that what they call a mapping?
P: 1,705
 Quote by hotcommodity I'm not quite sure. I know the range is in R^1, and the range is every value that f(x, y) can take on... What are the "strict quadrants?" Edit: Also, since the function takes an pair (x, y) from R^2, and turns it into R^1, is that what they call a mapping?
yes a function is a mapping. let me do this one for you and then give you another one

the domain for this function is all values for x and all values of except the point (0,0), i think you know why. and the range is $(-\infty,0)$ and $(0,\infty)$ notice unbounded below and open notation, the parenthesis instead of brackets, and the open and unbounded above, again the parenthesis instead of brackets. do you know why the range doesn't include zero?

try this now

$$f(x,y)=ln|x-y|$$

what is the domain and range of this function, describe it in words, set builder notation, and what it would look like on a graph
P: 436
 Quote by ice109 yes a function is a mapping. let me do this one for you and then give you another one the domain for this function is all values for x and all values of except the point (0,0), i think you know why. and the range is $(-\infty,0)$ and $(0,\infty)$ notice unbounded below and open notation, the parenthesis instead of brackets, and the open and unbounded above, again the parenthesis instead of brackets. do you know why the range doesn't include zero? try this now $$f(x,y)=ln|x-y|$$ what is the domain and range of this function, describe it in words, set builder notation, and what it would look like on a graph
First of all I appreciate the help.

A few questions, are you saying that the range of the function does not include zero, or that it does? And shouldn't it be for all z since z = f(x,y) ?

Ok, for $$f(x,y)=ln|x-y|$$

The domain of this function is the set of all x and y pairs such that x - y is greater than zero. The range of the function is the set z such that z = f(x,y), and z is greater than zero, and also x and y must fulfill the constraint for the domain. So D = {(x, y) | x > y}, and R = {z | z = f(x, y), x > y}. And graphically, I think it would exist in the first octant, and have a positive slope.
P: 1,705
 Quote by hotcommodity First of all I appreciate the help. A few questions, are you saying that the range of the function does not include zero, or that it does? And shouldn't it be for all z since z = f(x,y) ? Ok, for $$f(x,y)=ln|x-y|$$ The domain of this function is the set of all x and y pairs such that x - y is greater than zero. The range of the function is the set z such that z = f(x,y), and z is greater than zero, and also x and y must fulfill the constraint for the domain. So D = {(x, y) | x > y}, and R = {z | z = f(x, y), x > y}. And graphically, I think it would exist in the first octant, and have a positive slope.
yes the range of the first function doesn't contain zero. is the function zero anywhere?

look how i wrote the range of the previous function. it is pointless to write the range implicitly as you have. give me in interval notation. you're essentially saying that the range of the function f(x,y) is all values of f(x,y).

and about the domain graphically you're wrong

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