设S1=1+1/(1^2)+1/(2^2),S2=1+1/(2^2)+1/(3^2),S3=1+1/(3^2)+1/(4^2).Sn=1+1/[n^2+1/(n+1)^2].设S=√S1+√S2+√S

题目

设S1=1+1/(1^2)+1/(2^2),S2=1+1/(2^2)+1/(3^2),S3=1+1/(3^2)+1/(4^2).Sn=1+1/[n^2+1/(n+1)^2].设S=√S1+√S2+√S3+.+√Sn,则S=?(用含n的代数式表示,其中n为正整数）

答案

Sn=1+1/n^2+1/(n+1)^2=(n^4+2n^3+3n^2+2n+1)/(n^2*(n+1)^2)=(n*(n+1)+1)^2/(n^2*(n+1)^2)
故√Sn=√(n*(n+1)+1)^2/(n^2*(n+1)^2)=[n(n+1)+1]/[n(n+1)]
所以:
√S1=1+1-1/2
√S2=1+1/2-1/3
√S3=1+1/3-1/4
.
√Sn=1+1/n-1/(n+1)
s= 1+1-1/2 +1+1/2-1/3 1+1/3-1/4 +1+1/(n(n+1)))=n+[(1-1/2)+(1/2-1/3)+...+(1/n-1/(n+1))]=n+1-1/(n+1)