acosd returns a complex number

I'm trying to find acosd(15/9) in matalb but i'm getting 0.0000 +62.9458i, what dose that mean?

[Stephen23]

"I'm trying to find acosd(15/9) in matalb but i'm getting 0.0000 +62.9458i, what dose that mean?"

15/9

ans = 1.6667

What do you expect the arccosine to be for this value? Or, for that matter, for any value >1 ?

https://en.wikipedia.org/wiki/Inverse_trigonometric_functions