In the acute-angled triangle ABC, D is the intersection of the altitude passing through A with BC and Ia is the excenter of the triangle with respect to A. K is a point on the extension of AB from B, for which \angle AKI_a\equal{}90^\circ\plus{}\frac 34\angle C. IaK intersects the extension of AD at L. Prove that DIa bisects the angle ∠AIaB iff AL\equal{}2R. (R is the circumradius of ABC) geometrycircumcirclegeometric transformationreflectionsymmetryincenterangle bisector