# Which concepts in algebra/trig. have a domain and range

1. Jun 26, 2010

### land_of_ice

In algebra/trig. what kinds of problems have a domain and a range ?
Is this true for anything that can be graphed? Because alot of the time after you finish a problem in precalc. it then asks for the domain or range, so the question becomes, in mathematics, is it possible to find the domain and range, for ANY math problem, or what kind of areas of math does the concept of domain and range apply to ? To any math problem period?

2. Jun 27, 2010

### LumenPlacidum

All functions have domains and ranges. Functions are items to which you give them a value and they give you a value (usually these are numbers).

For example, all of the following are functions, and as such they have domains and ranges (which are also given, based on the typical real number analysis concepts):
f(x) = x (D = all real numbers, R = all real numbers)
f(x) = x+3 (D = all real numbers, R = all real numbers)
f(x) = 5x (D = all real numbers, R = all real numbers)
f(x) = 1/x (D = all real numbers except x=0, R = all real numbers except 0)
f(x) = x^2 (D = all real numbers, R = all non-negative real numbers)
f(x) = sqrt(x) (D = all non-negative real numbers, R = all non-negative real numbers)
f(x) = e^x (D = all real numbers, R = all positive real numbers)
f(x) = log(x) (D = all positive real numbers, R = all real numbers)
f(x) = sin(x) (D = all real numbers, R = the inclusive interval from -1 to 1)
f(x) = cos(x) (D = all real numbers, R = the inclusive interval from -1 to 1)
f(x) = tan(x) (D = all real numbers except those of the form n*pi/2 for n being an integer, R = all real numbers)
f(x) = arcsin(x) (D = the inclusive interval from -1 to 1, R = the inclusive interval from -pi/2 to pi/2)
f(x) = arccos(x) (D = the inclusive interval from -1 to 1, R = the inclusive interval from 0 to pi)

Only functions have domains and ranges because that's where their concepts make sense. For example, the domain of f(x) = x^2 is all the real numbers and the range is all the non-negative real numbers. That means that I can feed this function a real number (like, for example, 3) and it will spit out a non-negative real number (in this case, 3^2 = 9). I can't expect to get -3 out from this function no matter what I give it.

If I were talking about the function f(x) = arcsin(x), then the domain is the inclusive interval from -1 to 1 and the range is the inclusive interval from -pi/2 to pi/2. This means that I can feed it only a value between -1 and 1 (inclusive), like 0.5, and it will give me a response somewhere between -pi/2 and pi/2, in this case, pi/6.

3. Jun 27, 2010

### HallsofIvy

No, not only functions. Relations also have "domain" and "range".

4. Jun 27, 2010

### Martin Rattigan

A minor correction.

f(x) = tan(x) (D = all real numbers except those of the form (2n+1)*pi/2 for n being an integer, R = all real numbers)