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.