1、y=-1/2x²+x+3/2
=-1/2(x²-2x+1-1)+3/2
=-1/2(x-1)²+3/2+1/2
=-1/2(x-1)²+2
顶点:(1,2)
对称轴:x=1
2、y>0
-1/2(x-1)²+2>0
1/2(x-1)²