1) AC斜率=k, BC斜率=k/(-4)因AC垂直于BC,故k*k/(-4)=-1,得k=2或-22)由零点式,得y=a(x+1)(x-4)代入(0,k): k=-4a, 故a=-k/4因为开口向下,a<0,所以只能取k=2, 得:a=-1/2此时y=-1/2(x+1)(x-4)=-0.5(x^2-3x-4)=-0.5x^2+1.5x+2即a=-0.5, b=1.5, c=2