Question

What is the sum of a 6-term geometric sequence if the first term is 11, the last term is –11,264 and the common ratio is –4?

Answer 1

What formula do i use?

Answer 2

use Sn = a * ( 1 - r^n)
----------
1 - r

Answer 3

a - first term r = common ratio, n = no. of terms
you dont need to last trm

Answer 4

* you dont need to use the last term

Answer 5

Sn = 11 * ( 1 - (-4)^6) / 1-(-4)

Answer 6

ok ?

Answer 7

Let me write and solve it on my paper

Answer 8

ok

Answer 9

34,375

Answer 10

oh i got -9009

Answer 11

(-4)^6 = 4096 right?

Answer 12

Sn=11*(1-(-4)^6/(1-(-4)
Sn= 11* 5^6 / 5
Sn= 11* 15625 /5
Sn= 11* 3125
Sn= 34,375
thats how i got it maybe i did something wrong

Answer 13

yes - the second line is wrong
1 - (-4)^6 = 1 - 4096

only the (-4 )is taken to the power 6

Answer 14

note where the parentheses are

Answer 15

Okay i think i got it

Answer 16

in the formula the ^n refers to the r only

Answer 17

good

Answer 18

how do i solve this one

What is the sum of an 8-term geometric sequence if the first term is 10 and the last term is 781,250?

Answer 19

ok
i havent done one of these for a while....
the nth term of a geometric sequence = a r^(n-1)
so the eighth term = 10* r^(8-1)

10*r^7 = 781,250
r^7 = 78125
r = 5
so now we know a = 10, r = 5 and n = 8
and we can find the sum of 8 term using the formula

S8 = a * r^n - 1
-------
r - 1

notice that r^n comes before the 1 this time because r is positive

Answer 20

11607.?