The other tutor is right that 

But the rest is wrong because



Since he gave two answers, even though they are wrong, the
fact that he gave two answers instead of one shows that he
has forgotten that inverse trig functions are indeed 
functions.  Functions cannot have two output values for the 
same input value.  The range of the inverse cosine function 
is [0,p) 

So  is the only value.

Edwin