第1个回答 2013-01-03
a1+a3=a1+a1+2d=2a1+2d=6
a1+d=3 1
a11=a1+10d=21 2
2式-1式得
9d=18
d=2
a1=1
an=a1+(n-1)d=1+(n-1)*2=2n-1
bn=1/n(an+3)
=1/n(2n+3-1)=1/2n(n+1)=1/2*(1/n-1/(n+1))
Tn=b1+b2+b3+.........+bn
=1/2(1-1/2+1/2-1/3+..........+1/n-1/(n+1))
=1/2(1-1/(n+1))
=n/2(n+1)