English: Ellipse showing Pythagorean relation ${\displaystyle a^{2}=b^{2}+c^{2}}$. The image shows an ellipse constructed from the two foci F1 and F2, with the Pythagorean formula shown as the square of the semi-major axis (a) equal to the sum of the square of the semi-minor axis (b) and the square of (c). (c) is the distance between the center to any of the foci. (c) is also the eccentricity times the semi-major axis.