比较3a+1与2(a+1)05-3大小

2(a+1)^2-3-(3a+1)
=2a^2+4a+2-3-3a-1
=2a^2+a-2
=2(a^2+a/2+(1/4)^2-(1/4)^2)-2
=2(a+1/2)^2-1/8-2
=2(a+1/2)^2-17/8
(1)当2(a+1/2)^2-17/8＞0
(a+1/2)^2>17/16
a>-1/2+√17/4或a-1/2+√17/4或a=-1/2+√17/4或a