Question

Find S7 for the geometric series 2 + -6 + 18 + -54 +…

Answer

162

-486

1094

-4374

Answer 1

S7 = a1 * r^6

where a1 = first term and r = common ratio

Answer 2

r = second term / first term

Answer 3

So 2*-3^6?

Answer 4

oops sorry - i didnt read it properly- you need the sum of 7 terms not the 7th term

Answer 5

the formula for S7 is


a1 * r^7 - 1
-------
r - 1

Answer 6

Gimme a second...

Answer 7

The answer is 1094 :)

Answer 8

right

Answer 9

Thanks so much :)

Answer 10

its worth commiting that formula to memory

sum of n terms Sn = a1 * r^n - 1
-------
r - 1