Homework Help: Need help writing this as a sum

1. Jan 4, 2010

toasted

1. The problem statement, all variables and given/known data

Write as a series to represent the infinite sum and write the sum as a rational number.

0.71 + 0.7171 + 0.717171 + 0.71717171...

2. Jan 4, 2010

jgens

Are you sure that you've written the problem and/or sum correctly because the particular sum in question (0.71 + 0.7171 + 0.717171 + ...) diverges and consequently is not equal to any real number. Do you perhaps mean write the decimal 0.7171... as an infinite sum and then write this sum as a rational number?

3. Jan 9, 2010

danielatha4

$$\sum$$ Ak-1+.71*10-k-(k-1)

Starting on k=2, going to n, and assuming Ak=.71 when k=1

4. Jan 15, 2010

kashiark

$$\infty$$ i
$$\Sigma$$ $$\Sigma$$ 71*10^(-2a)
i = 1 a = 1

Of this, I am certain. a=1 should be under the second summation.

5. Jan 15, 2010

Staff: Mentor

I'm with jgens on this. As you have written it, this is a divergent series, so doesn't represent any real number.

Please give us the problem in its exact wording.

6. Jan 15, 2010

HallsofIvy

Okay, if you are certain that is the problem, then I am certain if does NOT converge.