Thread: Geometric Series challenging question

Suppose r and s are positive integers with no common factor and

1 - 1/2 + 1/3 - 1/4 + ... + 1/1339 - 1/1340 = r/s

Find a prime factor of r.

Have no idea how to solve this so please help.

Please explain it step by step clearly. Thanks.

So no one knows how to answer this?

Originally Posted by spriterman

So no one knows how to answer this?

Oh no, many of us know what needs to be done.
But why do you think we should do your work for you?

Here it is.

Now what can you do with that?

Doesn't that equation make the 1 negative? The starting term of the series is a positive.

Originally Posted by spriterman

Doesn't that equation make the 1 negative? The starting term of the series is a positive.

Are you saying that you do not know that

If so, no wonder you are having trouble.

Originally Posted by Plato

Are you saying that you do not know that

If so, no wonder you are having trouble.

Ok sorry, misread the equation.

But how did you get the equation the first place?

Also, this is not a geometric series...