数字帝国。@海离薇
变量代换 x＝t²＋4，dx＝2tdt，原式＝∫(1-->2) 2(t²＋6)/(t²＋4) dt＝2t ＋ 2arctan(t/2) | (1-->2)＝2＋π/2 - 2arctan(1/2)
