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###### Maths series help

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10 months ago

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

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)

Reply 2

10 months ago

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.

a, ar, ar^2, ar^3, ...

and the summation/sequence index would be on the exponent, not the base.

Reply 4

10 months ago

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.

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

Reply 6

10 months ago

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

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