y=3x+4=x^2得x1=-1或x2=4当x1=-1时y1=1当x2=4时y2=16A,B坐标为(-1,1),(4,16)设y=3x＋4 交y坐标轴于c，则OF=4S△AOB=OF*(x2-x1)/2=4*5/2=10
