If tangents PQ and PR are drawn from a point on the circle  x² + y² = 25 to the ellipse $\frac{{\mathrm{x}}^2}{4}+\frac{{\mathrm{y}}^2}{{\mathrm{b}}^2}=1,(\mathrm{b}<4)$,  so that the fourth vertex S of parallelogram PQSR lies on the circumcircle of triangle PQR, then eccentricity of the ellipse is