10-2-5 partial fractions into telescoping series
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So when doing this problem, we're going to figure out what
two terms would add to give me that 1 / n + 1 and plus two
term. So let's make that into a / n + 1 and b / n + 2. So we'd
have a * n + 2 + b * n + 1 equaling one if we let.
N equal -2 we'd have this first term canceling, so we'd get one
equaling b * -2 + 1, so B would be -1. If we let N equal -1,
we'd have one equaling a * -1 + 2 and the second term would
cancel, so we could see that a equal 1.
So a different way to write this would be a summation of N equal
1 to Infinity of 1 / n + 1 - 1 / n + 2. Now let's make sure that
I'm starting my index in the right spot. If I have one here,
I get 1 / 1 + 1 is 2 - 1 / 1 + 2 is 3. So if I have 1/2 - 1/3.
We'd get 3 - 2 / 2 * 3, which is our first term there, so we're
OK with that. Now we're going to look and realize by putting in a
couple terms. If I get 1 / 1 + 1 - 1 / 1 + 2 + 1 / 2 + 1 - 1 / 2
+ 2 plus...
This is going to be 1/2 -, 1/3 + 1/3 - 1/4. So it's going to be a
telescoping series. And we realized that the second term
and the first term of each of these parentheses are going to
cancel. So this is going to really equal 1/2 - 1 / n + 2.
So now what we need is we need to figure out what happens when
that N gets really, really large. So if N is going to
Infinity, we have one half 1 / n + 2. So that n + 2 is going to
get really, really big. But one over really, really big is
really, really small. So it's going to go to 1/2.