a3=a1+2d=3
a1+a4=a1+a1+3d=2a1+3d=2a1+4d-d=2*3-d=6-d=5
d=1
an=a3+(n-3)d
=3+n-3
=n若bn=1/an*an+1,求数列{bn}的前n项和Snbn=1/an*an+1 =1/n(n+1) =1/n-1/(n+1) Sn=b1+b2+..........+bn =1/1-1/2+1/2-1/3+.........+1/n-1/(n+1) =1-1/(n+1) =n/(n+1)