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## #1 2014-04-19 18:14:10

gourish
Member
Registered: 2013-05-28
Posts: 153

### Lagrange's interpolation polynomial

"A polynomial p(x) of degree two or less which takes values y0 y1 and y2 at three distinct values x0 x1 x2 respectively is given by {the la grange's interpolation polynomial}" use this data to answer
1.) A polynomial of degree 2 which takes values y0 , y0 , y1 at points x0 x0+t x1 t is not equal to 0 is given by?
2.)  A polynomial of degree 2 and "a" not equal to 0 or 1 then p(x) is....
3.) in last question if a tends to 0 then p(x) is given by

i don't know what this " la grange's interpolation polynomial is" can somebody tell me about it and explain a way to answer these question. i did not have this in my class so i find it hard to understand

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #2 2014-04-19 20:32:11

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

Hi gourish;

It is not a whole lot more than plugging in:

just remembering that j ≠ k in 2).

Are you restricted to only the Lagrange interpolation formula?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #3 2014-04-19 21:31:39

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

yeah only that... plus i don't know what the Lagrange's interpolation formula is... so can you help me with that

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #4 2014-04-19 22:55:12

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

The formula is best demonstrated with a live example.

Want to see one?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #5 2014-04-20 00:21:54

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

yeah for sure.... quite excited to see it

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #6 2014-04-20 00:38:29

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

Actually, it is a primitive idea and I am wondering why they still teach it like they do but here is the example.

You are familiar with a set of xy coordinates that are represented as ordered pairs.

{(0,1),(1,5),(2,18),{3,85)}

We have 4 points so that means we can fit a 3rd order poly through them and get an exact fit. This is much easier to see if you use other methods but for right now follow my signature.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #7 2014-04-20 00:40:52

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

ok i get it.. but does it mean that it's the other way of saying "look there is a relation between two sets and i can get so and so number of functions out of the relations set"?

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #8 2014-04-20 00:44:15

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

but can you just show me how to use it with an example

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #9 2014-04-20 00:45:40

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

Actually what it is saying is that you will need n simultaneous equations to fit n points and the result will be an (n-1)th order polynomial but lets not get caught up with jargon, lets see how we can determine the answer using Lagrange.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #10 2014-04-20 00:46:47

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

yeah that's what i want to learn....

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #11 2014-04-20 00:48:22

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

Compute f(0.3) for the data
x
0
1
3
4
7
f
1
3
49
129
813
using Lagrange's interpolation formula (Analytic value is 1.831)

this is what i found on the net. can you show me how to find answer to this question using Lagrange's formula

reference for the actual question :http://mat.iitm.ac.in/home/sryedida/public_html/caimna/interpolation/lagrange.html

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #12 2014-04-20 00:52:44

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

That problem is bigger than the one in post #6 so it is more work but it it is not more informative. It will also require more time to post, I therefore recommend the smaller problem first.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #13 2014-04-20 01:01:23

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

ok yeah let's go with the smaller one then....

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #14 2014-04-20 01:01:55

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

I will need some time to post the latex...

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #15 2014-04-20 01:05:45

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

okay i can wait.. you take your time and post....

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #16 2014-04-20 01:42:06

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

Where f(2) = 18 and x1 = 1.

Using the formulas;

we get:

Adding all the l's up using 2) in post #2:

So the Lagrangian polynomial for that data is

Now do you see why I would never use this method?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #17 2014-04-20 02:53:26

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

i would rather hit my head on the desk then use this formula... it's better to guess on the answer from the choices....

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #18 2014-04-20 03:01:27

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

I would think that the whole process could be automated.

But much easier is just solving a linear set of equations.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #19 2014-04-20 03:04:36

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

yeah it kicks out the mystery of things in finding the solution thanks

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #20 2014-04-20 03:07:41

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

You understand how it was derived?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #21 2014-04-20 03:12:57

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

no i am sorry the signs used are quite complicated for me. i never really came across these before...

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #22 2014-04-20 03:15:10

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

i will go through them again and try to understand how it was derived....

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #23 2014-04-20 03:17:08

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

Hi;

I meant the simultaneous set in post 18.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #24 2014-04-20 03:18:41

gourish
Member
Registered: 2013-05-28
Posts: 153

### Re: Lagrange's interpolation polynomial

i did not understand the l 2 and l 3 part and sorry i still couldn't understand how it was derived

"The man was just too bored so he invented maths for fun"
-some wise guy

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## #25 2014-04-20 03:25:47

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Lagrange's interpolation polynomial

If you want to fit a cubic to 4 data points perfectly you would start with the general form of a cubic.

and now just substitute the data points for x and y.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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