设y=kx,k<0
则：直线l与y=x+3的交点为C(3/(k-1),3k/(k-1)）
S△ABO=1/2|AO||BO|=(1/2)*3*3=4.5
S△AOC=1/2|AO||Cy|=(1/2)*3*3k/(k-1)=4.5k/(k-1)
(1)S△AOC:S△ABO=1:3
==>4.5k/(k-1)*3=4.5
==>k=-1/2
==>L:y=-1/2x
(2)S△AOC:S△ABO=2:3
==>4.5k/(k-1)*(3/2)=4.5
==>k=-2
==>L:y=-2x