# Domain of a function?

• Sep 13th 2008, 12:59 PM
Kilo
Domain of a function?
How do you find the domain of a fuction such as f(x)=(x+sin x)/cos x?
• Sep 13th 2008, 02:21 PM
o_O
Hint: For what values of $x$ will cause the denominator be equal to 0? Remember, you can't divide by 0.
• Sep 13th 2008, 02:29 PM
Shyam
Quote:

Originally Posted by Kilo
How do you find the domain of a fuction such as f(x)=(x+sin x)/cos x?

since denominator, cos x is zero when $x= \frac{n \pi}{2}, n\in \mathbb{I}$

So, domain is all real values except this value.

Domain = { $x\in \mathbb{R}/ x\ne\frac{n \pi}{2}, n\in \mathbb{I}$}

or you can write this way:

Domain $= \mathbb{R} - \frac{n \pi}{2}, n\in \mathbb{I}$
• Sep 13th 2008, 02:36 PM
o_O
Quote:

Originally Posted by Shyam
since denominator, cos x is zero when $x= \frac{n \pi}{2}, n\in \mathbb{I}$

So, domain is all real values except this value.

Domain = { $x\in \mathbb{R}/ x\ne\frac{n \pi}{2}, n\in \mathbb{I}$}

or you can write this way:

Domain $= \mathbb{R} - \frac{n \pi}{2}, n\in \mathbb{I}$

Not quite. For example, take n = 2. $\cos \left(\frac{2\pi}{2}\right) = \cos \pi = -1$

I think you mean: $x = \frac{\pi}{2} + n\pi \ \ n \in \mathbb{Z}$
• Sep 13th 2008, 03:17 PM
Kilo
Quote:

Originally Posted by Kilo
How do you find the domain of a fuction such as f(x)=(x+sin x)/cos x?

Okay, there's another problem that basically is this one....but a wee bit different....

f(x)=(x+sin x)/cos x, -3pi/2 (less than equal to) x (less than equal to) 3pi/2

Yeah, now I'm really confused....
• Sep 14th 2008, 01:53 AM
Moo
Quote:

Originally Posted by Kilo
Okay, there's another problem that basically is this one....but a wee bit different....

f(x)=(x+sin x)/cos x, -3pi/2 (less than equal to) x (less than equal to) 3pi/2

Yeah, now I'm really confused....

For what x between $-\frac{3 \pi}{2}$ and $\frac{3 \pi}{2}$ is $\cos(x)=0$ ?

The formula is as follows :
$\cos(a)=\cos(b) \implies \text{either } \left\{\begin{array}{ll} a=b+2k \pi \\ a=-b+2k' \pi \end{array} \right. \text{ where k and k' } \in \mathbb{Z}$

$\cos \left(\tfrac \pi 2\right)=0$ so in fact, it will give $\cos(x)=0 \Longleftrightarrow x=\frac \pi 2+k \pi$ (which is o_O's formula)

This will give you all the possibilities for $-\frac{3 \pi}{2} \le x \le \frac{3 \pi}{2}$, that is... try to fit values for k such that x remains between these two values.
• Sep 14th 2008, 08:09 AM
Shyam
Quote:

Originally Posted by o_O
Not quite. For example, take n = 2. $\cos \left(\frac{2\pi}{2}\right) = \cos \pi = -1$

I think you mean: $x = \frac{\pi}{2} + n\pi \ \ n \in \mathbb{Z}$

Oh, yes, I realised it later. There was a mistake. Actually it should be,

Domain={ $x\in \mathbb{R}/ x\ne\frac{(2n-1) \pi}{2}, n\in \mathbb{I}$}

or you can say

Domain= $\mathbb{R} - \frac{(2n-1) \pi}{2}, n\in \mathbb{I}$

because, cos x is zero for odd multiples of $\frac{\pi}{2}$
• Sep 14th 2008, 08:18 AM
Shyam
Quote:

Originally Posted by Kilo
Okay, there's another problem that basically is this one....but a wee bit different....

f(x)=(x+sin x)/cos x, -3pi/2 (less than equal to) x (less than equal to) 3pi/2

Yeah, now I'm really confused....

you see, cos x is 0 when x =odd multiple of $\frac{\pi}{2}$

so, $x \ne \frac{-3\pi}{2},\frac{-\pi}{2},\frac{\pi}{2},\frac{3\pi}{2}$

Now, you try to write the domain.
• Sep 14th 2008, 01:48 PM
Kilo
(Bow)Thanks everyone! It's starting to make sense to me now! (Nod)
• Sep 15th 2008, 07:11 AM
log(xy)
i know this thread is kinda done with, but cant the domain really just be any real number? in algebra we know that the domain of a function can be any real number, so long as it has only one range, but thats besides the point of the fact that shouldnt the answer be any real number?