y= x^2-3x+7
f'(x) = 2x - 3 =0
x = 3/2
f(3/2) = 9/4 - 9/2 + 7
= 19/7
顶点(3/2,19/7)
用配方法
y= x^2-3x+7
= (x-3/2)^2 +19/4
Miny = y(3/2) = 19/4