Let g ( x ) be a quartic ( 4 th degree) polynomial such that g ( n ) = n 3 − 2 n for n = 0 , 1 , 2 , 3 , 4 .
Find
4 5 ∫ 0 4 g ( x ) d x
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@Otto Bretscher Nice solution! I've converted your comment into this solution.
In future, if you have such gems to share, you can mention me to convert it into a solution :)
Using the Poyla Form,
n 3 = 6 ( n 3 ) + 6 ( n 2 ) + ( n 1 )
and 2 n = ( n 4 ) + ( n 3 ) + ( n 2 ) + ( n 1 ) + ( n 0 ) f o r 0 ≤ x ≤ 4 a n d x i s a n i n t e g e r
So, n 3 − 2 n = − ( n 4 ) + 5 ( n 3 ) + 5 ( n 2 ) − ( n 0 ) f o r 0 ≤ x ≤ 4 a n d x i s a n i n t e g e r
∫ 0 4 ( − 2 4 x 4 + 1 2 1 3 x 3 − 2 4 1 1 x 2 − 1 2 7 x − 1 ) d x = [ − 1 2 0 x 5 + 4 8 1 3 x 4 − 7 2 1 1 x 3 − 2 4 7 x 2 − x ] 0 4 = 4 5 1 9 0 6
4 5 1 9 0 6 × 4 5 = 1 9 0 6
Good approach.
Or use Boole's Rule :
2 ( 7 f ( 0 ) + 3 2 f ( 1 ) + 1 2 f ( 2 ) + 3 2 f ( 3 ) + 7 f ( 4 ) ) = 2 ( − 7 − 3 2 + 4 8 + 6 0 8 + 3 3 6 ) = 1 9 0 6
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Also good!
Don't ignore error term
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Well, it's a quartic, so that there is no error term; the error term involves the sixth derivative.
Then Boole's rule approximating the integral of f ( x )
So it's just a coincidence that the approximation that you've found is exactly equal to the answer? How do you know the integral is exactly equal to 1906?
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As I explained below, the error term involves the sixth derivative, so that the formula is exact for polynomials up to degree 5.
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@Otto Bretscher – Got a proof?
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@Pi Han Goh – There is a proof in my linear algebra text ;) This formula belongs to the class of Newton-Cotes formulas, together with trapezoidal, Simpson's etc. The proof is a bit technical, but it involves nothing more than the mean value theorem. I will find you a reference tomorrow.
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@Otto Bretscher – Looking forward to it. Thanks!
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@Pi Han Goh – There is some good stuff here
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@Otto Bretscher – Ohhhh Simpsons rule = specific case of Newton-Cotes Formulas???? I didn't know that! Good to know
Can you write this as a separate solution? It's a very neat one-liner.
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I don't think he got the question correct...
I must confess that I did not actually attempt the problem... I'm a busy man ;) It did not look like an interesting problem to work out. I often just look at solutions to see whether I can learn something. When I saw Joel's long and awkward solution, I thought I write a one-liner to remind people of the elegant Newton-Cotes formulas.
did you get the question correct
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Or use Boole's Rule :
2 ( 7 f ( 0 ) + 3 2 f ( 1 ) + 1 2 f ( 2 ) + 3 2 f ( 3 ) + 7 f ( 4 ) ) = 2 ( − 7 − 3 2 + 4 8 + 6 0 8 + 3 3 6 ) = 1 9 0 6