How to use arc tan to find an angle?

[hasheesh786]

in physics we were doing a problem that essentially involved vectors and in the end we were left with 2 displacements. how would i find the angle between them if it was not a right triangle? i asked someone and they said that i could find it using artan(Ry/Rx) ...R being one of the displacements

 

[HallsofIvy]

If Ry and Rx are the y and x components of a vector, then there is a right angle between them- the x and y axes are at right angles. So, yes, Arctan(Ry/Rx) is the angle the vector makes with the positive x axis. (Since tangent is periodic, arctan is multivalued. Arctan is the "principal" value between -90 and 90 degrees. Make sure that is what you want.)

 

[hasheesh786]

If Ry and Rx are the y and x components of a vector, then there is a right angle between them- the x and y axes are at right angles. So, yes, Arctan(Ry/Rx) is the angle the vector makes with the positive x axis. (Since tangent is periodic, arctan is multivalued. Arctan is the "principal" value between -90 and 90 degrees. Make sure that is what you want.)

so if the R value is 218.684158367 and the other displacement is 187 how would i find the angle between them using the arc tan?