For x^2/a^2 + y^2/b^2 = 1, compute S1 = x1^2/a^2 + y1^2/b^2 - 1. S1 < 0: inside, S1 = 0: on the ellipse, S1 > 0: outside. The centre (0,0) gives S1 = -1 (inside). Any point (x1,y1) satisfying the inequality x1^2/a^2 + y1^2/b^2 < 1 is inside.