On 07/06/2015 11:20, tim..... wrote:

"John Rumm" wrote in message
news
On 06/06/2015 19:07, tim..... wrote:

"Farmer Giles" wrote in message
...
On 06/06/2015 11:15, tim..... wrote:

"Tim Streater" wrote in message
.. .
In article , Tim Watts
wrote:

Prove that: n^2-n-90=0

For n = -9 or n = 10, easy enough to do in one's head.

That's a solution

not a proof

tim




Correct, it is not a proof - neither was my answer earlier.

This is, though: 6/n x 5/n-1 = 1/3

-- 30/n^2-n = 1/3

-- 90 = n^2-n -- 0 = n^2-n-90

but it's still not a proof in the context of a maths question, which is

prove that n = 10 (from the information in the question, not from the
equation)

There is no requirement to "prove" n=10

You are misunderstanding the way that "proof" in mathematics works.

Well actually that was the accusation I was levelling at you[1] ;-)
Although I suspect we are both singing from the same hymn sheet - but
something has been lost in translation.

[1] Due to your response suggesting that one needs to prove n = 10

You have been asked for a proof, so in order to get the marks you first
have to find something that you can "prove".

Indeed.

Farmer Giles correctly provided a proof IMHO - i.e. derived the required
equation algebraically from the source equations extracted from the
problem description.

Simply using the equation to solve n=10 isn't it, because that is not a
proof.

Agreed.

And as the equation resolved down to n=10 the thing that you have, that
can be proved, is that n=10 is, in fact, the solution to the narrative
part of the question.

Not really - you can make the proof without ever finding n (which can
also be -9, a valid solution to the equation, although makes no sense in
the context of the problem)

Thus there becomes a requirement to prove that n=10 solves the sweet
problem, because that's all you have that you can use as the result of a
mathematical proof.

I am not sure about that - you can't prove that n = 10, since its not
the only valid answer. The best you can say about 10 is that it is one
possible solution for n.

Finding any number of solutions is not usually an adequate mathematical
proof (unless proving a negative of course! I suspect that Andrew Wiles
may be a tad upset if you can find integer values for a, b, & c where
a^3 = b^3 + c^3 )


(and finding that n=10 would be finding a solution, not providing a
proof)

Correct, so it's can't be the solution to the question that gets you the
marks

Which is what I was saying!



--
Cheers,

John.

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