A(0,2),B(4,3)y=ax^2+bx+cx=0,c=2x=4,y=316a+4b+2=316a+4b-1=0......(1)y=a(x+0.5b/a)^2+2-0.25b^2/a抛物线的对称轴x=-0.25b/a2+0.25b/a=|1|=±1......(2)(1),(2):a= b=