an=(1+2+...+n)/n
=(1+n)*n/2n
=(1+n)/2
a(n+1)=(n+2)/2
bn=1/an·a(n+1)=4/(n+1)(n+2)=2/(n+1)-2/(n+2)
S(bn)=b1+b2+...+bn
=(2/2-2/3)+(2/3-2/4)+...+(2/(n+1)-2/(n+2))
=1-2/(n+2)
=n/(n+2)
an=(1+2+...+n)/n
=(1+n)*n/2n
=(1+n)/2
a(n+1)=(n+2)/2
bn=1/an·a(n+1)=4/(n+1)(n+2)=2/(n+1)-2/(n+2)
S(bn)=b1+b2+...+bn
=(2/2-2/3)+(2/3-2/4)+...+(2/(n+1)-2/(n+2))
=1-2/(n+2)
=n/(n+2)