y=x^2-5x+6的单调区间
y=x^2-5x+6
a=1,b=-5,c=6;
对称轴等于:x= -b/2a= --5/2 =2.5
a>0,抛物线开口向上,
(-无穷 , 2.5) 递减
(2.5, +无穷)递增