Question

How do you find the sum of the first 20 terms of the sequence 1, -3, 9,...?

Answer 1

`-871696100`

Explanation:

Formula of Sum of Geometric Series = `Gn=a (1-r^n)/(1-r)`
where `a=1, r=-3/1=-3, n=20`
Therefore
`Gn=a (1-r^n)/(1-r)=G20=1[(1-(-3)^20)/(1-(-3)]]`
`G20=1[(1-(3)^20)/(4]]` (negative sign within bracket was omitted as even power gives positive result)
`G20=1[(1-3486784401)/4]`
`G20=1[(-3486784400)/4]`
`G20=-871696100`