Question

Find S15 for 1 + 1.5 + 2.25 + 3.375 + …
i know the answer is 873.8, but can someone please give me an easier formula to get this answer?

Answer 1

@bibby

Answer 2

there's just the standard formula we used in the previous question

r= 1.5
\(\huge a_n=a_1*r^{n-1}\)
\(\huge a_n=1*1.5^{n-1}\)

Answer 3

oops

Answer 4

are they asking for the 15th term or what

Answer 5

yes

Answer 6

yeah, same formula as last time. how'd you get 873.8

Answer 7

by dividing the number by 2 then adding the same number to the answer we got after dividing... example 2.25/2=1.125 then 1.125+2.25=3.375

Answer 8

ok, first of all to find the common ratio we divide a term by the term before it
\(\huge \frac{1.5}{1}=\frac{2.25}{1.5}=\frac{3.375}{2.25}=1.5\)

Answer 9

ok that makes it easier! thank you!

Answer 10

so what's our next step?

Answer 11

multiplying each number by 1.5?

Answer 12

assuming they want the 15th term, no. we just plug into the formula

Answer 13

whats n?

Answer 14

nth term means the term we want to find. if we want \(s_{15}\) n=15

Answer 15

but the answer that gives me is 291.9

Answer 16

how do you know what the answer should be

Answer 17

that would mean we're finding the partial sum, not the n-th term

Answer 18

\(\huge S_n=\frac{a_1(1-r^n}{1-r}\)

Answer 19

ohhh ok! that gave me the answer i needed! thank you(:

Answer 20

npnp p.s. your smiley face is backwards
:)*