The focus of an ellipse x2a2+y2b2=1\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1a2x2+b2y2=1 is at (c,0)(c, 0)(c,0). How is ccc calculated?
c=a+bc = a + bc=a+b
c=a2+b2c = \sqrt{a^2 + b^2}c=a2+b2
c=a2−b2c = \sqrt{a^2 - b^2}c=a2−b2
c=a−bc = a - bc=a−b