Let $AA_1, BB_1$ and $CC_1$ be the altitudes of an acute-angled triangle $ABC.$ $AA_1$ meets $B_1C_1$ in a point $K.$ The circumcircles of triangles $A_1KC_1$ and $A_1KB_1$ intersect the lines $AB$ and $AC$ for the second time at points $N$ and $L$ respectively. Prove that a) The sum of diameters of these two circles is equal to $BC,$ b) $\frac{A_1N}{BB_1} + \frac{A_1L}{CC_1}=1.$