keep just positive or just negetive answer of acos?

I have arccos and I want to keep just negative answer of it? how can I do it?(there are two answer)

Answer 1

[jonas]

If A is a vector with your answers. Remove non-negatives by

A(A>=0)=[]

[Najmeh Eskandari]

no it is inverse cos and has two answers. i want to keep positive answer and think it is not a vector.

[jonas]

I don't understand. What function did you use? The fact that you want to keep only one value implies that you have two, hence my simple fix.

MATLAB's acos however only returns a single positive solution. If you want a negative solution, just multiply by -1, as cos(x)=cos(-x)

"For real elements of x in the interval [-1,1], acos(x) returns real values in the interval [0,pi]." - the doc