Posted by mathstudent on Tuesday, February 10, 2009 at 3:20pm.
I read from my textbook:
If S is the infinite series 1 + x + x^2 + x^3 + ...
Then Sx = x + x^2 + x^3 + x^4 + ... = S  1
So, S = 1/(1x)
I follow what that logic, but it still doesn't make sense.
The way I see it, if you plug any real number > 1 into x, S will be infinity which does not equal 1/(1x) at all...
For example, if I plug the constant 10 in for x,
The infinite series "1 + x + x^2 + x^3 + ..." will be infinity
yet 1/(1x) will equal 1/9.
Can someone explain this?

math  bobpursley, Tuesday, February 10, 2009 at 3:45pm
So if x>=1, both series do not converge, but diverge, and head for greater sums. But if x<1, they converge.
Now, if x<1, does S=1/(1x) ?

math  Damon, Tuesday, February 10, 2009 at 3:48pm
If S is the infinite series 1 + x + x^2 + x^3 + ...
Then Sx = x + x^2 + x^3 + x^4 + ... = S  1
well, say you terminate at 4 terms as you did:
xS = x + x^2 + x^3 + x^4
S =1+x + x^2 + x^3
then
xSS = x^4 1
and
S (x1) = (x^41)
S = (x^41) / (x1)
which is what you had except I have that x^4
No matter how many terms you take, xS will always have a sum bigger than S by that x^n at the end so it is not just the 1

math  Damon, Tuesday, February 10, 2009 at 3:49pm
By the way, if x is <1, then that x^n at the end will go to zero so it will all work.

math  mathstudent, Tuesday, February 10, 2009 at 4:36pm
Thanks guys. I wrote a simple computer program that verifies that S = 1/(1x) holds when 0 < x < 1 (hence the series converges), but not when x > 1 (and the series diverges). That wasn't clear in the textbook. Thanks for the help.
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