Mathf.Cos error

Why is that Mathf.cos is not accurate??
Here is my code:

function Update () { print(Mathf.Cos(135)); }

unity will print -0.9960 but is should be -0.7071?!
I try it on my calculator?
Is this a bug?
by the way I’m using unity 3.4.0f5

Mathf.Cos uses radians, not degrees. Try

Mathf.Cos(135 * Mathf.Deg2Rad);

and see what you get!