Let the circumcentre of a triangle with vertices \( A(a,3) \), \( B(b,5) \) and \( C(a,b) \), \( ab > 0 \) be \( P(1,1) \). If the line \( AP \) intersects the line \( BC \) at the point \( Q(k_1,k_2) \), then \( k_1 + k_2 \) is equal to:
\( 2 \)
\( \dfrac{4}{7} \)
\( \dfrac{2}{7} \)
\( 4 \)