Okay that makes sense. So to make sure, if the point were ##r = 2xi +3yj##, I could use (0,3R) or if I take ##dy##, I could use (3R,0) right?
so in this case, the denominator becomes ##(x^2+9R^2)##?
In essence, we have to keep one of the coordinate as a constant, am i right?
I can write it as...