代入方程
-4=a﹙-1+h）²+k ①
-2=a(1+h)²+k ②
①-②
-2=a(-1+h)²-a(1+h)²=a(-1+h-1-h)(-1+h+1+h)=-4ah
ah=1/2
x=1,y最值,
∴h=-1
a=-1/2
k=-2
y=-1/2(x-1)²-2