Question

Help!!!! What is the sum of the geometric sequence 1, -6, 36, ... if there are 7 terms?

Answer 1

Help???

Answer 2

sn = a1(1 - r^n) / (1 - r)
n = 7 (your looking for total of sum of numbers to the 7th term)
a1 = first term = 1
r = common ratio = -6

now lets sub our info in the formula...
s7 = 1(1 - (-6^7) / (1 - (-6)
s7 = 1(1 - (- 279936) / (1 + 6)
s7 = 1(1 + 279936) / 7
s7 = 1(279937)/7
s7 = 279937/7
s7 = 39991 <-- your answer