2a＞0 2a+1＞0 2a+2＞0 2a+3＞0 2a+4＞0 2a+5＞0 2a+6＞0 2a+7＞0 2a+8＞0
2a+9＞0
｛2a＞0 2a+1＞0 ｛2a+1＞0 2a+2＞0 ｛2a+2＞0 2a+3＞0 ｛2a+3＞0 2a+4＞0 ｛2a+4＞0 2a+5＞0 ｛2a+5＞0 2a+6＞0 ｛2a+6＞0 2a+7＞0 ｛2a+7＞0 2a+8＞0
｛2a+8＞0 2a+9＞0 ｛2a+9＞0 2a+10＞0 虽然是应付但绝对正确（家庭作业吧）