# Easy Domain

1. Mar 29, 2008

### Calixto

What would the domain of y = sqrt(cosx) be in mathematical terms. I know that it is all the reals that lie in the first and fourth quadrant of the unit circle, but how would you express that in mathematical terms?

2. Mar 29, 2008

### nicksauce

Something like
$$D=\{x|x\in[\frac{(4n-1)\pi}{2},\frac{(4n+1)\pi}{2}]\}$$
where n is any integer

3. Mar 30, 2008

### unplebeian

You first need to define the domain of x.
If x is +60 for example then cos(x)>0 but if it is -60, then it is <0. sqrt of that is an imaginary number which is not defined.

So basically for x>0 domain is (0,1)

I would like to know how Calixto got his answer as I may be wrong.