a1＝7,a﹙n＋1﹚＝3an^4﹙两边取对数﹚
问题描述：
a1＝7,a﹙n＋1﹚＝3an^4﹙两边取对数﹚
求通项公式
答
令bn = log an则b(n+1) = log a(n+1) = log 3an^4= log 3 + 4log an= log 3 + 4bn可以解得[b(n+1) + (log 3)/3 ] = 4[bn + (log 3)/3 ]bn + (log 3)/3 = 4^(n-1)(b1 + (log 3)/3)剩下的就是代入的工作了...答案是什么an = 7^(4^(n-1)) * 3^[(4^(n-1)-1)/3]就是7的（4的n-1次方）乘以3的3分之（4的n-1次方减1）次方