# Maths series help

Help on this seq and series q

I don’t get how to get part a because when I did it I did the sum of the first using this equation as it’s geometric

Sn= a(1-r^n) / 1-r
But a is the first term and it’s given as k=0
When I sub k equals zero into k cubed I get 0 as the first term therefore a=0
Now when I sub a=0 into geometric series sum equation shown above… I get 0 but that isn’t correct
(edited 10 months ago)
Thats not geometric. A geometric would be ar^k. Its cubic, so k^3
Original post by mqb2766
Thats not geometric. A geometric would be ar^k. Its cubic, so k^3

Is it arithmetic ???
I don’t get why it’s not geometric because geometric is a sequence that times right?
I mean it’s
K x k x k
(edited 10 months ago)
You really should have covered basic geometric, arithmetic (linear) and quadratic sequences at gcse. This is cubic as its k^3, whch is pretty much the definition of a basic cubic. A geometric would go
a, ar, ar^2, ar^3, ...
and the summation/sequence index would be on the exponent, not the base.
Original post by mqb2766
You really should have covered basic geometric, arithmetic (linear) and quadratic sequences at gcse. This is cubic as its k^3, whch is pretty much the definition of a basic cubic. A geometric would go
a, ar, ar^2, ar^3, ...
and the summation/sequence index would be on the exponent, not the base.

We didn’t do GCSEs because of covid… and we got topic lists of what to revise… so that’s why I don’t know how to do it, sorry.thanks for the help
This is easier than you think. The difference between the sum of n terms and the sum of (n-1) terms is just the final nth term.
Original post by Matureb
This is easier than you think. The difference between the sum of n terms and the sum of (n-1) terms is just the final nth term.

Thank you, it very kind of you for not judging my ability