# Homework Help: Is this equation correct

1. Jan 8, 2013

### utkarshakash

1. The problem statement, all variables and given/known data
Is this equation correct for permissible values of x

$tan^{-1}|tan x| = |x|$

2. Relevant equations

3. The attempt at a solution
I assume LHS to be θ.
Then tanθ=|tanx|
The original equation becomes
$tan^{-1}tan \theta = |x|$

2. Jan 8, 2013

### tiny-tim

hi utkarshakash!

(what do you mean by "permissible"? )

wouldn't it be easier to start by saying |tanx| = tan|x| ?

3. Jan 8, 2013

### HallsofIvy

No, that is not correct because tangent is not a "one-to-one function". For example, if $\theta= 5\pi/4$ then $tan(\theta)= tan(5\pi/4)= 1$ so that $tan^{-1}(tan(\theta))= tan^{-1}(tan(5\pi/4))= tan^{-1}(1)= \pi/4$, not $5\pi/4$. Since everything is positive, the absolute value is irrelevant.

4. Jan 8, 2013

### Millennial

Even if so, it is still wrong, as tangent can be negative for positive values of x. For instance, $\tan(7\pi/4)=-1$. It is easy to observe from here that $|\tan(7\pi/4)|\neq \tan|7\pi/4|$, as $1\neq-1$.

5. Jan 9, 2013