further maths series help

Im a bit stuck on how to get the answer to the question linked in the correct format. I know that you break it down into the parts that you can see in the capture 2 i linked, but I can't seem to manipulate it into the correct format.

Any help would be appreciated!

(edited 1 year ago)

Original post by speghettipasta

Im a bit stuck on how to get the answer to the question linked in the correct format. I know that you break it down into the parts that you can see in the capture 2 i linked, but I can't seem to manipulate it into the correct format.

Any help would be appreciated!

If you work back from the answer you shold have
1/3 (n^3 - a^2 n)
as its a difference of two squares, so expand what you have (if you cant see any obvious factors) and stuff should cancel. If not, you may have a mistake and post your working.

(edited 1 year ago)

Reply 2

potentialgcsepro

9

Original post by speghettipasta

Im a bit stuck on how to get the answer to the question linked in the correct format. I know that you break it down into the parts that you can see in the capture 2 i linked, but I can't seem to manipulate it into the correct format.

Any help would be appreciated!

theres a difference of 2 squares so when you expand it some will cancel out.

Reply 3

mqb2766

21

Original post by speghettipasta

ah okay thank you, I think I have made a mistake then.
my working is linked below

Pulling 1/6 out seems to go wrong.
Note you could try n=1 to make sure your answer matches the original sum. In fact you could also determine "a" using that value as well.

Reply 4

speghettipasta

OP

9

Original post by mqb2766

Pulling 1/6 out seems to go wrong.
Note you could try n=1 to make sure your answer matches the original sum. In fact you could also determine "a" using that value as well.

i have figure it out thank you!
if you take out the 1/3 then it simplifies to 1/3n(n^2 - 25) which is a difference of two squares.

Reply 5

mqb2766

21

Original post by speghettipasta

i have figure it out thank you!
if you take out the 1/3 then it simplifies to 1/3n(n^2 - 25) which is a difference of two squares.

Yes. Pulling 1/6 out (correctly) was the obvious thing to do as it keeps ithings as integers, but pulling 1/3 out keeps it in line with the thing you want to show.

(edited 1 year ago)

further maths series help

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