求救partial fraction

DreamHospital

given that Un=1/(n^2-n+1)-1/(n^2+n+1).
find SN=2N
             Σ         Un
             n=N+1                 in terms of N.

find a number M such that SN<10^(-20) for all N>M.

中文

antimatter

看不懂……………………………………………………………………………………………………………………………………

DreamHospital

可以了

puangenlun

Second term of U(n)=- First term of U(n+1)

==>S(N)=1/((N+1)^2-(N+1)+1)-1/((2N)^2+(2N)+1)

For S(N)<10^(-20),
........,
N>..............=M