Question

高二数学数列问题数学高手进急在线等

Answer 1

1、
a3= 7……①
a5+a7=26……②
∵{an}是等差数列
∴a3+a7=2·a5 …… ③
解得：a3=7，a5=11，a7=15
∴d=(a7 - a3)/2 = 2
a1=a3-2d=3
∴an = a1+(n-1)d=2n+1

2、 (难道你的题目被打了马赛克，看不清楚啊！！！是 bn = 1/(an² - 1)吗？)
bn = 1/(an² - 1)
=1/[(an + 1)(an - 1)]
=1/[2n(2n+2)]
=1/[4n(n+1)]
=(1/4)·1/[n(n+1)]
=(1/4)·[(1/n) - 1/(n+1) ]

∴Tn = (1/4)·[(1 - 1/2) + (1/2 - 1/3) + (1/3 - 1/4) + …… + ( 1/n - 1/<n+1> )]
=(1/4)·[1 - 1/(n+1)]
=(1/4)·[n/(n+1)]
=n/[4(n+1)]

Answer 2

a1+2d=7 ,a1+5d=13,d=2,a1=3,an=2n+1 ,Sn=n2+2n,第二问bn=?那里看不清楚