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**zetafunc.****Guest**

Is this formula okay to use for linear interpolation (finding the root of a function in an interval [a,b])?

However, in one mark scheme (in a Jan 2007 paper) I saw them use this formula, but with absolute values.

Thanks.

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Hi;

Have you tried that formula for something simple like

It is reasonably close to a root. You would need the root to be bracketed between a and b.

Like all formulas that are derived from theory it sometimes fails laughingly.

Try it out on f(x) = x^2

Give it a = -1 and b = 1.

**In mathematics, you don't understand things. You just get used to them.**

**I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.**

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**zetafunc.****Guest**

Thanks for the reply.

I've been using it for every linear interpolation question I've encountered in the FP1 exam. They recommend the similar triangles method, but it's a bit tedious because you have to do some manipulation to get the root on its own... but with this formula it's a bit easier. However, I can't find anything about this formula online...

It appears that they take the absolute value of f(a) or f(b) (replace the f(a) and f(b)s with |f(a)| and |f(b)|) in these older past papers though... for example, for f(x) = ln(x) + x - 3, they said to use linear interpolation to find a root between 2.0 and 2.5.

f(2) gives you a negative value (about -0.3069) and f(2.5) gives you a positive value (about 0.4163). But they've taken the absolute values of both and used those for f(a) and f(b) instead...

Maybe it's better if I use the similar triangles method but then check my answer using this shorter formula?

**zetafunc.****Guest**

Sorry for not seeing your f(x) = x[sup]2[/sup] example.

Sorry, forgot to specify, it doesn't work if f(a) = f(b) since denominator would be 0.

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Hi;

Other than that it seems to be okay, what is it derived from?

**In mathematics, you don't understand things. You just get used to them.**

**I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.**

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**zetafunc.****Guest**

Not sure... I found it on the internet somewhere on some discussion forum so it's probably pretty unreliable because someone could have just made it up without justification. But I figured out the official formula that's accepted;

Will probably worry about the derivation until after the exam has finished, as they're not allowed to ask us any proof questions on that topic (only proof by induction questions will come up for series, sequences and proving some function produces some multiple of a number, luckily).

Exam is on the morning of 30th Jan, last exam... really want it out of the way. Maths is so boring when you're just doing past papers all the time.

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Hi;

That one works pretty good on the example given, but I still do not recognize it. I have rearranged it:

**In mathematics, you don't understand things. You just get used to them.**

**I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.**

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**zetafunc.****Guest**

Thanks, I will write the re-arrangement out in the exam! (after I write the non-re-arranged formula out first)

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Hi zetafunc.;

Okay, and good luck on your exam. Let me know how you do.

**In mathematics, you don't understand things. You just get used to them.**

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**zetafunc.****Guest**

Had the exam today and didn't even need this formula because it didn't come up! Strangely it was an unusually easy paper... I think I got everything correct but there were some slight issues that worried me and may have cost me a couple of marks. One question asked about using Newton-Raphson twice on some function and give the final value to 3 d.p., but didn't know if you should round after doing it the first time. You got the same answer if you rounded or not, though. And then there was plotting complex roots on an Argand diagram... I did them as points not lines! I thought, for example, 1 + i is just the point (1,i), whereas |1 + i| is a line joining (0,0) and (1,i). I'm probably wrong, I don't know...

The paper was 90 minutes long, most people left after about 40 minutes! I stayed the full 90 minutes and checked everything as relentlessly as I could but didn't find anything. Unfortunately, because it was so easy, grade boundaries will be high and tiny mistakes could be incredibly costly...

Only got 1 more chemistry exam to go and then no more until May...

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Sounds like you did well.

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**zetafunc.****Guest**

Hmm, I just checked my FP1 textbook and sometimes they're drawn as dots and sometimes they're drawn as lines... but in the FP2 textbook they're always lines. I think lines might be the correct thing to do, then... looks like I lost a couple of marks.

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

I would think they are points and not vectors.

I left some stuff over in your other thread on matrix transformations.

**In mathematics, you don't understand things. You just get used to them.**

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**zetafunc.****Guest**

bobbym wrote:

I would think they are points and not vectors.

Ah, that makes sense... thanks. Most people in my class drew lines though.

Summer will be stressful though so I hope I can take it one step at a time. I have started some revision for those since I have a lot to do, but only baby steps.

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Why will summer be stressful? Summer is a time of barbecues and parties in castles thrown by Dukes and Barons.

**In mathematics, you don't understand things. You just get used to them.**

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**zetafunc.****Guest**

bobbym wrote:

Why will summer be stressful? Summer is a time of barbecues and parties in castles thrown by Dukes and Barons.

Summer up until mid-July at least. 2 months of exams from mid-May to early July (around 22 of them). Then I can take maybe a month off and relax and prepare for the next year. If everything goes to plan I should have 4 exams in Jan 2013 and 2 or 3 in June 2013, which would give me time to just sit back and enjoy maths for a bit rather than train to pass an exam. Then of course there is the issue with this girl in my class but that's something else... anyway.

I am supposed to get the results on March 8th. All mark schemes, grade boundaries, exam statistics etc come out on March 1st. I wish it wasn't so far away!

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

There is always a girl in class to distract you. Nothing can turn a mans mind faster into a big bucket of mush than a girl.

**In mathematics, you don't understand things. You just get used to them.**

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**zetafunc.****Guest**

bobbym wrote:

There is always a girl in class to distract you. Nothing can turn a mans mind faster into a big bucket of mush than a girl.

I suppose... she sits in front of me in all of my exams, literally within touching distance! It's a distraction but I don't mind...

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Same thing happened to me. She was DDG and sat in front me. I guess it is some kind of test that God gives all guys.

Just do not dip her hair in the inkwell!

**In mathematics, you don't understand things. You just get used to them.**

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**zetafunc.****Guest**

bobbym wrote:

Same thing happened to me. She was DDG and sat in front me. I guess it is some kind of test that God gives all guys.

That's the real test we have to pass, I guess... what does DDG stand for?

I don't know why but I keep trying to find ways to talk to her and end up wasting my time for nothing.

**zetafunc.****Guest**

DDG = drop-dead gorgeous?

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

DDG means drop dead gorgeous. They start off being a little bit unsure of themselves and cautious the first week or two that they know you are sitting behind them. Trying to figure you out. But as their confidence grows, watchout!

**In mathematics, you don't understand things. You just get used to them.**

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,820

hi zf

Y U NO BECOM MEMBAR???

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Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**zetafunc.****Guest**

bobbym wrote:

DDG means drop dead gorgeous. They start off being a little bit unsure of themselves and cautious the first week or two that they know you are sitting behind them. Trying to figure you out. But as their confidence grows, watchout!

Hmm... in the exam hall I sit behind her but in the classroom we're at completely opposite corners... the furthest possible distance apart!

It's sort of a strange dynamic. I'm in a class with year 13 kids and I am a year 12 (they're all 1 year older than me). I've talked to everyone in my class except for three people. She is one of those three. I do sort of feel like my brain is going mushy or recognising similarities that mean nothing. She's a year older than me but we got identical GCSE grades, identical module scores, we seem to get stuck on the same questions and we're both quick, and we seem to talk the same way. Doesn't really mean anything though. Oh look, I've just spent half an hour thinking about it instead of doing something more productive! Ah well...

**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Hi;

It is normal do not worry about it. Happens to everybody so we all wasted and are wasting are time. The playing field is level.

You are 12? Meet anonimnystefy he is 14.

**In mathematics, you don't understand things. You just get used to them.**

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