But I still don't see how adding an infinite amount of numbers can give you a finite sum.
Well that shouldn't be too difficult. Let us examine a simple fraction: 1 / 9. You will agree that this number is a number that truly exists: if you cut a pizza in nine equal pieces we call such a slice 1 / 9 th part. Oddly, if we try to represent this fraction as a decimal fraction something 'strange' happens: (I know this is done differently in the English-speaking countries, but this is how I was taught to do it)
9 / 1.0 \ 0.111
9
---
10
9
---
10
9
---
1
And the division is still not finished.... We see that if we try to actually divide 1 by 9 by doing a "tail division" (as it is literally called in Dutch) a very special number develops: a zero with a repeating series of 1 digits behind the fractional point. You can try it out for your self, but you will see that the series never ends, and every extra division by 9 will give you another 1 in the series. The fractional part consists of an infinite series of 1's.
Now lets rewrite part of the number we saw above in a different form: if I asked you to rewrite the number 0.11111 (five ones) as a sum of fractions where each fraction's numerator is required to be one, you might come up with the following:
1 1 1 1 1
---- + ----- + ------ + ------- + -------- = 0.11111
10 100 1000 10000 100000
What happens here is that the numerator is constant, that is 1, and that the denominator is a power of 10, and for each digit in the fraction increases to the next power of 10.
If we go back to the original example of the fraction 1 / 9 I already you that the same number in decimal form has an infinite number of 1-decimals. So the number 0.111 ... (where the 1's repeat ad inifinitum) is equal to 1 / 9. When we rewrote 0.11111 (five ones) as a sum of fractions, I showed you how the series of 5 digits behind the fractional point can be seen as a sum of a series of fractions that have 1 for numerator and and increasing power of 10 for denominator. Combining the two shows that:
1 1 1 1 1 1 1
---- + ----- + ------ + ------- + -------- + ... + -------- = -
10 100 1000 10000 100000 10^n 9
The expression 10^n should be read as 10 to the power of the largest number you can think of, namely n. And if you have thought of that number just multiply it a few times more by 10 so that it gets even larger, which is in effect the same as doing a couple of more divisions in the division example.
I hope the above is understandable enough to show how adding an infinite amount of numbers can still produce a finite sum.
(c)