Tim finds a new way to write the nth term of an infinite geometric series
Tim
worked on a few infinite series like
with
partial sums 1/3, 4/9, 13/27, 40/81, … which goes to
½
. Don asked Tim how to get from 81 to 40 in the 4th partial sum. His
first discovery was that if you add the numerator and the denominator of the
previous fraction you get 40. That was fine. Was there another way to generalize
the nth partial sum? Tim came up with the following, which Don had never seen
before:
which he generalized to
… and this works for any of these infinite geometric series with numerator 1. Don asked Tim if he could do a similar thing for a series starting with, say 2/5. Tune in next week to see what he does.
Great job, Tim !
In the meantime, Don did the following in
Mathematica for the same series above.
Sum[3^x/3^4,{x,0,4-1}]
40/81
which is the 4th partial sum with n=4
that Tim did above.
Sum[3^x/3^3,{x,0,3-1}] with n=3, we get 13/27
The nth partial sum
Sum[3^x/3^n,{x,0,n-1}]
The limit of the right side above as n->∞, equals 1/2;
done in mathematica,Limit[Sum[3^x/3^n,{x,0,n-1}],n->
∞]1/2
The nth partial sum starting with 2/5 (this took Don quite a while to figure out how to write this sum and then write it as a Mathematica
command):Sum[(2^x*5^(n-x))/5^n,{x,1,n}]
1/3* 2^(1 + n)* 5^(-n)*(-1 + (5/2)^n)
The 3rd partial sum starting with 2/5 is
Sum[(2^x*5^(3-x))/5^3,{x,1,3}]
78/125 which is what he got above.
The limit of the series starting with 2/5 is 2/3
Limit[Sum[(2^x*5^(n-x))5^n,{x,1,n}],n->∞]
2/3