一次函數(shù)y=ax+b的圖象經(jīng)過(-2,1),
1=-2a+b
2a-b=-1
4a-2b+3=2(2a-b)+3=2*(-1)+3=1
所以,y=ax^2-bx+3過定點(diǎn)(2,1)
y=ax^2+bx+3=a(x+b/2a)^2+(12a-b^2)/4a
2a-b=-1,b=2a+1
對(duì)稱軸:x=-b/2a=-(2a+1)/2a=1
2a=-(2a+1),a=-1/4
所以,a=-1/4時(shí),對(duì)稱軸是x=1
頂點(diǎn)縱坐標(biāo)=(12a-b^2)/4a
=(12a-(2a+1)^2)/4a
=(-4a^2+8a-1)/4a
=(-a+2-1/4a)
≥2*√(-a*(-1/4a))+2
=1+2
=3
其頂點(diǎn)縱坐標(biāo)的最小值為3
可見,敘述正確的有2條