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# (STEP) Sequences Help Please! watch

1. I have a formula in the form f(n) = ax^n + by^n
And I want to know how I find the common difference/ common ratio between each term?

I have worked out what both a,b,x and y are, if that helps!
a = 1/sqrt(3) , b = -1/sqrt(3)
x = (1+sqrt(3))/2 , y = (1-sqrt(3))/2

Also, if this helps too:
F(0) = 0, F(1) = 1, F(2) = 1, F(3) = 2

I started by putting:
F(n) = ax^n + by^n
=1/(sqrt(3) [x^n - y^n]
And therefore
F(n+1) = ax^n+1 + by^n+1
=1/(sqrt(3) [x^n+1 - y^n+1]

But I'm not sure how to figure out the common difference/ common ratio, since x gets x times bigger, and y gets y times bigger, each term. But I can't find a value that does all in one go...

I'm being askedto evaluate the sum to infinity of f(n)/2^n+1 so my first attempt is to try and find how much bigger f(n) gets each term, but I can't xD
2. (Original post by ComputerMaths97)
I
But I'm not sure how to figure out the common difference/ common ratio, since x gets x times bigger, and y gets y times bigger, each term. But I can't find a value that does all in one go...

I'm being askedto evaluate the sum to infinity of f(n)/2^n+1 so my first attempt is to try and find how much bigger f(n) gets each term, but I can't xD
I would forget about looking for a common difference or ratio, as there isn't one for the whole term.

It may help to split the sequence into two.

f(n) = ax^n + by^n

Let g(n) = ax^n
Let h(n) = by^n

Both nice geometric series.

And f(n) = g(n) + h(n)

...
3. Also: Why not post the question? Context is always useful.

And BODMAS (or whatever precedence mnemonic you prefer) is not just an empty acronym. About 50% of what you've written is clearly wrong because of precedence rules. (E.g. you mean but what you've actually written says ).

We can guess what you mean from context but those guesses are never 100% certain and if I have to make ten 90%-likely guesses about what you mean the likelihood is I'll be wrong at some point.

In particular, I really don't know if you are trying to find or

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Updated: October 26, 2015
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