What Does Cis^-1 (x) Mean and How is it Related to Cis(x)?

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Homework Statement

What does cis^-1 (x) mean
like you know how there is sin^-1 (x) and sin(x) applying that concept with cis(x) what does that mean?

Note that is not a spelling mistake I'm not trying to say cos^-1 (x) I'm indeed saying cis^-1 (x) with an "i"

I also wander what does cis stand for like cos is cosine sin is sine etc. how do I say cis...

like I was thinking ok

cis(x) = cos(x) + i sin(x) = e^(ix)
what does cis^-1(x) mean

kind of stuck here...

Thank You

Homework Equations

The Attempt at a Solution

 

[jegues]

Since cis(x) is defined in the following manner,

$$cis(x) = cos(x) +isin(x) = e^{ix}$$

Simply take the inverse of cis(x),

$$cis^{-1}(x) = \pm cos^{-1}(\frac{x^{2} + 1}{2x})$$