数列求解
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Sn=n^2+2n
n=1, a1=3
for n>=2
an = Sn - S(n-1)
=2n-1+2
=2n+1
bn = 1/[an.a(n+1)]
= (1/2)[1/(2n+1) -1/(2n+3)]
Tn = b1+b2+...+bn
= (1/2)[1/3 -1/(2n+3)]
n=1, a1=3
for n>=2
an = Sn - S(n-1)
=2n-1+2
=2n+1
bn = 1/[an.a(n+1)]
= (1/2)[1/(2n+1) -1/(2n+3)]
Tn = b1+b2+...+bn
= (1/2)[1/3 -1/(2n+3)]
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