Please help me with series&sequencing problem?

Question

Consider the sequences: 5, 10, 20, 40, 80.
a^20=2621440
s^20=?

please help ive tried multiple times, im so frusterated!!!!

Answer 1

Sn = a ( ( r^n -1) / (r- 1) )
where a is first term and r is common ratio .
in the given question a = 5 and r = 2 and n = 20

S^20 = 5 ( 5^20 - 1) / ( 5 - 1)
= 5 * ( 5^20 -1 ) / 4 = 11920928550780

Answer 2

My complications are that i'm 18 and that i don't have a job, I left college at 13-14 and not went lower back, I even haven't any GCSE's and not something else, i'm able to't get a job to assist my mum who's suffering a week to keep a roof over our head. i hit upon it not problem-free to fulfill new human beings, or to even leave the homestead as i think that I actually have Agoraphobia and a threat Asperger's Syndrome - the picture of my brother. Yeah existence is not problem-free, yet i'm attempting to pull myself together the most proper i'm able to, one step at a time & all that.. only experience fantastically valueless from time to time, suicidal ideas have cropped up better than once.. yet i don't think of I actually have the balls to kill myself, or i'd have achieved it already.

Answer 3

I take it (a^20) is just the 20th number in the sequence, since each term is double the last one.

If (s^20) is the sum of all of them, it's just going to be (a^21)-5, your first term.

Answer 4

a_n= 5*2^(n-1) as r=2
S_n= 5( 2^n-1)
so S_20= 5*1,048,575 =5,242,875