π/6≤x+π/6≤7π/6
如果b>0
最小值为：a-b
最大值为：a+(b/2)
{a+(b/2)=2/5
{a-b=1/4
b=1/10
a=7/20
如果b<0
最大值为；a-b
最小值为；a+(b/2)
{a-b=2/5
{a+(b/2)=1/4
b=-1/10
a=3/10