- #1
mr_coffee
- 1,629
- 1
We learned in class how to find the sum of any geometric sequence with the following formula:
Let x = Sum Of Geometric Sequence;
x = [Take mythical next term - real term]/(ratio - 1);
The real term is the first term of the sequence and the mythical next term would be the next term for example after this sequence:
ex:
1+2+2^2+2^3 +...+2^n = ?
first term is : 1
mythical next term is: 2^(n+1)
ratio: 2
when I say ratio I mean what you multiply each term to get to the next and you can see its 2 here.
x = [2^(n+1)-1]/(2-1)
So that would be the formula to sum up the sequence...so that one was easy but this is the one that i don't understand:
2^n + 2^(n-1) x 3 + 2^(n-2) x 3^2 + 2^(n-3) x 3^3 + ... + 2^3 x 3^(n-3)
I don't know if I'm getting the ratio right or not...
I see each term is getting multiplied by 3, the first term is just 3^0 = 1, then 3^1, 3^2...
I see n is being decremented by 1 each time and its a power of 2, so would that be: 1/2^n
So the ratio i figured out is: 3/2^n
Now the mythical next term I got is: 2^4 x 3^(n-4)
or do i not mess with the 2^3? and leave it as 2^3 x 3^(n-4) ?
The 1st real term is: 2^n
So here is the formula i come out with:
Sum of geometric sequence = [ [2^4 x 3^(n-4)] - 2^n ]/[(3/2^n)-1]
I'm testing for n = 3, n = 4, and n = 5 to see if itss right...
for n = 3
2^3 + 2^2 x 3 + 2 x 3^2 = 38
now if i plug 3 into the formula:
[2^4 x 3^(-1) - 2^3 ] / (3/(2^3) - 1) = 64/15 which isn't 38...
Do you see what I'm doing wrong? I think I'm screwing up on the ratio and the mythical next term, the next term in the sequence, any help would be great. I have to do it the way the professor showed us with that general formula:
Let x = Sum Of Geometric Sequence;
x = [Take mythical next term - real term]/(ratio - 1);
Thanks!
Let x = Sum Of Geometric Sequence;
x = [Take mythical next term - real term]/(ratio - 1);
The real term is the first term of the sequence and the mythical next term would be the next term for example after this sequence:
ex:
1+2+2^2+2^3 +...+2^n = ?
first term is : 1
mythical next term is: 2^(n+1)
ratio: 2
when I say ratio I mean what you multiply each term to get to the next and you can see its 2 here.
x = [2^(n+1)-1]/(2-1)
So that would be the formula to sum up the sequence...so that one was easy but this is the one that i don't understand:
2^n + 2^(n-1) x 3 + 2^(n-2) x 3^2 + 2^(n-3) x 3^3 + ... + 2^3 x 3^(n-3)
I don't know if I'm getting the ratio right or not...
I see each term is getting multiplied by 3, the first term is just 3^0 = 1, then 3^1, 3^2...
I see n is being decremented by 1 each time and its a power of 2, so would that be: 1/2^n
So the ratio i figured out is: 3/2^n
Now the mythical next term I got is: 2^4 x 3^(n-4)
or do i not mess with the 2^3? and leave it as 2^3 x 3^(n-4) ?
The 1st real term is: 2^n
So here is the formula i come out with:
Sum of geometric sequence = [ [2^4 x 3^(n-4)] - 2^n ]/[(3/2^n)-1]
I'm testing for n = 3, n = 4, and n = 5 to see if itss right...
for n = 3
2^3 + 2^2 x 3 + 2 x 3^2 = 38
now if i plug 3 into the formula:
[2^4 x 3^(-1) - 2^3 ] / (3/(2^3) - 1) = 64/15 which isn't 38...
Do you see what I'm doing wrong? I think I'm screwing up on the ratio and the mythical next term, the next term in the sequence, any help would be great. I have to do it the way the professor showed us with that general formula:
Let x = Sum Of Geometric Sequence;
x = [Take mythical next term - real term]/(ratio - 1);
Thanks!