How does one find the value of tan theta that =1/3

I tried to work it out using a triangle and inverse trig functions

If arctan(1/3)=theta then theta = tan(1/3)

that means we should be able to draw a right angle triangle where the side opposte theta is 1 and the side adjacent is 3

this would make the hypotenuse sqrt(10)

however when I use arcsin and arccos to find the angles I get both angles near 18 which would imply that the triangle is not a right triangle.

Am I on the right track at all?

perhaps my calculator should be in radian mode? is that the what im missing? I'm not sure.