20.在 Delta ABC 中,角A,B,C的对边分别为a,b,c, (sin )^2A+(sin )^2B=|sin C-dfrac (2sqrt {3)}(3)sin Asin B|sin C-|||-(1)求角;-|||-(2)若 =sqrt (19), 边AB上的中线 =dfrac (sqrt {7)}(2). 求边a,b的长.