How to get a monotonous result of 'acos'?

When I try to calculate the inverse cosine of a periodic x, for instance:

a = -10 : 0.1 :10;
x = cos(a);

And then try to use 'acos(x)', I get:

So my question is, how can I get the original * a*? Or more precisely, how can I get a monotonous result of acos(x)?

(Hope I have expressed myself clearly enough...> <)

[random09983492]

Think about how the input to a cosine works... ie cos(0) = cos(2*pi)