Hi, dear all, hope your guys allow me to ask this tricky question. Refer to the attachment, i would like to derive the principal radii of an ellipse, the final equation and figure is provided in Stephen Timoshenko Theory of plates and shells 2nd edition. consider the ellipse has form b^2*x^2+a^2*y^2=(a^2)*(b^2) may i know how can i get the correct variable for the red color highlighted length? because the final equation is so complex, i expect it is not y=2b+unknown. I use the center as focus origin, the final equation i derive is not correct.as the focus origin move. In others words , how to get the length for the focus origin as highlighted in red color?