Math Is Fun Forum

Hello, i have a very silly question but i am kind of rusty in maths lately. So the question is: we have a cyrcle, and we know the radius and the center which is the origin (0,0). We also know a point on the circle, lets say (1,1). We need to calcualate the angle θ. I know that sin θ = opposite/hypotinuse. But how can we calculate the θ angle and not the sin θ. I am using a 3d software that calculates sin and cos.  Thanks a lot in advance.