A picture paints a thousand words..... A worksheet helps much more.
So: you should ALWAYS attach your worksheet.
Other than that: Fricas (the new symbolic engine in Prime 6) may be less capable in solving 4th order equations than MuPad. Switch symbolic engine to the 'classic' symbolic engine and see what happens.
Trying to explain why muPad also adds -acos(...) to its solution and why this is correct in my opinion.
In mathematics arccos is NOT the inverse of the cos function, but rather is the inverse limited to the range from 0 to pi. The inverse of the cos function would have an infinity number of values for every single argument and therefore is not a function in the mathematical definition.
To solve the equation cos(x)=C requires x = arccos(C) but also x = - arccos(C) OR x = 2 pi - arccos(C). And of course you may add any integer multiple of 2 pi to any of these two "main" solutions.
friCAS seems to have its own interpretation and I definitely prefer the correct answers of muPad:
