# Homework Help: Find Domain Of: √(log1/5(1/Sin5x))

1. Apr 8, 2012

### Plutonium88

1. The problem statement, all variables and given/known data
√(log1/5(1/Sin5x))

Log to the base 1/5*(1/Sin5x)

FIND THE DOMAIN

I have an answer to this solution, I Just need some help with understanding it :(... and also could someone tell me if info is missing.

f(x) = √(log1/5(1/Sin5x))

Restrictions:

1/sin5x ǂ 0

sin5x ǂ 0

5x = ∏n
xǂ (∏n)/5

√(log1/5(1/Sin5x)) ≥ 0
(square both side)
(log1/5(1/Sin5x)) ≥ 0

1/sin5x ≤ 1

Sin5x ≥ 1

**** Sin M ≤ 1 ***** (this is refferring to the slope)

Sin5x = Sin(∏/2 + 2∏n)

5x = = ∏/2 + 2∏n
x = ∏/10 + 2∏n/5

ans: XE(∏/10 + 2∏n/5]

anyways i would appreciate if some one could help me understand this solution better. <3

2. Apr 8, 2012

### HallsofIvy

I assume you mean log base 1/5:
$$\sqrt{log_{1/5}(\frac{1}{sin(5x)})}$$
Yes, the sin(5x) in the denominator cannot be 0. Sine sin(x) is 0 for x any multiple of $\pi$, that means 5x cannot be as multiple of $\pi$:$x\ne n\pi/5$ for any integer n, as you say.

But also, logarithm is only defined for positive numbers so in addition to $sin(5)\ne 0$, we must have sin(5x)> 0. $sin(\theta)$ is positive for $0< x\le \pi$, $2\pi< x< 3\pi$, etc. We can write that as $2n\pi< x< (2n1)\pi$ for any integer n.

I don't know why you are looking at "$\sqrt{log_{1/5}(1/sin(5x)}\ge 0$". That is a statement about the value or range of the function and has nothing to do with the domain. Rather, what is important is that in order that a square root exist, the argument must be non-negative. And $\log(x)\ge 0$ if and only if $x\ge 1$. So we must have $log_{1/5}(1/sin(5x))\ge 1$ which means that we must have $1/sin(5x)\ge 1/5[/tex] which, since we have already required that it be positive, is the same as [itex]5\ge sin(5x)$. For what x is that true?

3. Apr 9, 2012