# Help on Series.

1. Oct 5, 2003

### mrnoll

hi.

how can I prove that if A is a converges and B diverges

that the Sum of these series (A +B) diverges..

( A = a1 + a2 + a3 + ...
B = b1 + b2 + b3 + ...) if the series start from n=1

2. Oct 5, 2003

### Kalimaa23

Limit of a sum is the sum of the limits.
The convergence of the series is determined by the row of partial sums.
Therefore, if both rows converge, the sum of both will too.

3. Oct 5, 2003

### HallsofIvy

In order to apply Dimitri Terryn's suggestion, you will also have to note that if C=(A+ B), then Sum(B)= Sum(C- A). Assume both Sum(C) and Sum(A) converge. What does that tell you about Sum(B)?

4. Oct 5, 2003

### mrnoll

Hi,

I dont think it´s that easy actually.
Because what I forgot to say was that these series go from
n=1 to n=infinity.

And in infinity normal rules dont always apply like SumA + SumB = Sum(A+B)

I need a proof... :)

5. Oct 6, 2003

### phoenixthoth

it is that easy because what the infinite series converges to is defined to be the limit of the sequence of partial sums.

6. Oct 8, 2003

### HallsofIvy

In this case they do. If Sum(A) and Sum(B) exist then
Sum(A+ B) exists and is equal to Sum(A)+ Sum(B).

I had assumed you were talking about infinite series since otherwise it wouldn't make sense to talk about "convergent" and "divergent".