Only at a at the moment, I know x coordinate would be 0 but not sure how I'd find the y coordinate.
I'm guessing the equation for the circle that B and A lie on is
x^2+y^2=\lambda^2
so maybe
y=\sqrt{\lambda^2-x^2}
is the half that B lies on, I don't know where to go from there 🤷♂️