Question

Suppose a1=12−12,a2=23−13,a3=34−14,a4=45−15,a5=56−16. a) Find an explicit formula for an: . b) Determine whether the sequence is convergent or divergent: . (Enter "convergent" or "divergent" as appropriate.)

Answer 1

I'm going to assume you meant to write fractions (because if $a_n$ are all non-negative integers, the series would clearly diverge), so that

$a_1=\dfrac12-\dfrac12$

$a_2=\dfrac23-\dfrac13$

$a_3=\dfrac34-\dfrac14$

and so on.

a. If the pattern continues as above, we would have the general term

$a_n=\dfrac n{n+1}-\dfrac1{n+1}=\dfrac{n-1}{n+1}$

b. Note that we can write $a_n$ as

$a_n=\dfrac{n-1}{n+1}=\dfrac{n+1-2}{n+1}=1-\dfrac2{n+1}$

The series diverges by comparison to the divergent series

$\displaystyle\sum_{n=1}^\infty\frac1n$