1,y=1/2x²-4x-3,-b/2a=4/(1/2×2）=4,（4ac-b²）/4a=-22/2,所以对称轴是x=4,顶点坐标（4,22/2）.2,y=3x²+2x -b/2a=-2/6=-1/3 (4ac-b²）/4a=-4/12=-1/3.所以对称轴为x=-1/3 顶点坐标(-1/3 -1/3).
用公式法求下列抛物线的顶点坐标和对称轴
1,y=1/2x²-4x-3,-b/2a=4/(1/2×2）=4,（4ac-b²）/4a=-22/2,所以对称轴是x=4,顶点坐标（4,22/2）.2,y=3x²+2x -b/2a=-2/6=-1/3 (4ac-b²）/4a=-4/12=-1/3.所以对称轴为x=-1/3 顶点坐标(-1/3 -1/3).