Mar 3, 2008 · So you want the centroid of half the ellipse (the part where x≥0). By symmetry, its y-coordinate will be 0. For the x-coordinate, the formula is \frac1A\int_0^axl\,dx, where l is the length of the cross-section at x. This is given by l=\frac{2b}a\sqrt{a^2-x^2}, since y goes from -\frac{b}a\sqrt{a^2-x^2} to +\frac{b}a\sqrt{a^2-x^2}. Also, A is ...