Find the sum of the first 11 terms of the geometric sequence. Use the formula for the sum of the first n terms of a geometric sequence. 6,-30,150,-750,....

Question

 The sum of the first 11 terms of the geometric sequence is 

Answer 1

The formula is a(1 - r¹¹)/(1-r) where a = 6 and r = -5

 

This amounts to (1 + 5¹¹) which my calculator gives as 1 [phone]

Answer 2

a₁=6, a₂=-30, a₃=150, a₄=-750,...

r=-30/6=-5

 

S₁₁=a₁(1-r¹¹)/(1-r)=6(1-(-5)11)/(1+5)=(1+5*5¹⁰)=48828126