Assume that you wish to draw an ellipse with a semi-major axis of a = 20 cm and an eccentricity of e = 0.5. How long would your string need to be? (Hint: think about the case where e = 0, i.e., a circle). Given that the eccentricity of an ellipse is c/a, where c is the distance of each focus from the center of the ellipse, how far apart would the thumbtacks (at the focii) need to be?