Question
given the geometric series 1+2+4+8 find the sum between (inclusively) the 5th term and the 15th term.
what I did I'm sure its wrong but I tired like that :
the 5th term is 16 because the r is *2 in each term and the a5th term is :
15=1*2^n
15=2^7.5
S15=1{(2)^7.5)-1}/2-1
S15=180
is that correct, I'm sure is not ,help please!!!
what I did I'm sure its wrong but I tired like that :
the 5th term is 16 because the r is *2 in each term and the a5th term is :
15=1*2^n
15=2^7.5
S15=1{(2)^7.5)-1}/2-1
S15=180
is that correct, I'm sure is not ,help please!!!
Answers
Steve
Since
a = 1
r = 2
S4 = 1(2^4-1)/(2-1) = 15
S15 = 2^15-1 = 32768
So, the sum from S5 to S15 = S15-S4 = 32753
a = 1
r = 2
S4 = 1(2^4-1)/(2-1) = 15
S15 = 2^15-1 = 32768
So, the sum from S5 to S15 = S15-S4 = 32753