I have this series: ∑ n = 1 ∞ 4 ( n + 1 ) ( n + 2 )

Question

I have this series:      ∑          n      =      1        ∞        4          (      n      +      1      )      (      n      +      2      )

Sage Mcdowell · Accepted Answer

One way to show convergence of the first series, is to first note that

\frac{4}{(n + 1) (n + 2)} < \frac{4}{(n + 1)^{2}}

for all n. Then write

\sum_{n = 1}^{\infty} \frac{4}{(n + 1)^{2}} = 4 \sum_{n = 2}^{\infty} \frac{1}{n^{2}} .

Presumably, you already know that the latter series converges, and so you can apply the comparison test, to show that

\sum_{n = 1}^{\infty} \frac{4}{(n + 1) (n + 2)}

converges. The other one can be treated in a similar way.
To find the sum, write

\frac{1}{(n + 1) (n + 2)} = \frac{A}{n + 1} + \frac{B}{n + 2},

and solve for A and B (this is called a partial fractions decomposition). Then you should be able to determine the n'th partial sum of this series explicitly, and easily evaluate the sum (these types of series will be called telescopic in your textbook, for obvious reasons). The other one can be treated in a similar way, but the computations will be a bit more complicated.

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