SOLUTION: Find the 20th term of a geometric sequence whose third term is 1 and common ratio is -3. I tried doing a₂₀= 1 (-3) ²⁰ ⁻ ³ a₂₀= (-3)¹⁷ The answer I got was

Algebra ->  Sequences-and-series -> SOLUTION: Find the 20th term of a geometric sequence whose third term is 1 and common ratio is -3. I tried doing a₂₀= 1 (-3) ²⁰ ⁻ ³ a₂₀= (-3)¹⁷ The answer I got was       Log On


   

Question 1188248: Find the 20th term of a geometric sequence whose third term is 1 and common
ratio is -3.
I tried doing
a₂₀= 1 (-3) ²⁰ ⁻ ³
a₂₀= (-3)¹⁷
The answer I got was -129140163 but it doesn't feel correct. I think my equation is wrong but I don't know how else to put it.
Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
the second term is -(1/3)
the first term is (1/9)
The 20th term is a1*r^(n-1)=(1/9)(-3)^19, which is what you have.
brute force
4th term=(-3)
20th term will be negative
3^16=43046721
3^20 will be -129140163.
In this case, brute force can be done, and it takes a little while, but it works.