Integrate the unknown function please

Calculus Level 5

Let g ( x ) g\left( x \right) be a quartic ( 4 th { 4 }^\text{th} degree) polynomial such that g ( n ) = n 3 2 n g\left( n \right) ={ n }^{ 3 }-{ 2 }^{ n } for n = 0 , 1 , 2 , 3 , 4 n=0,1,2,3,4 .

Find

45 0 4 g ( x ) d x 45\int _{ 0 }^{ 4 }{ g\left( x \right) dx }


The answer is 1906.

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2 solutions

Otto Bretscher
Mar 27, 2016

Or use Boole's Rule :

2 ( 7 f ( 0 ) + 32 f ( 1 ) + 12 f ( 2 ) + 32 f ( 3 ) + 7 f ( 4 ) ) = 2 ( 7 32 + 48 + 608 + 336 ) = 1906 2\left(7f(0)+32f(1)+12f(2)+32f(3)+7f(4)\right)=2(-7-32+48+608+336)=\boxed{1906}

@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 :)

Calvin Lin Staff - 5 years, 2 months ago

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Thanks @Calvin Lin ! Will do!

Otto Bretscher - 5 years, 2 months ago
Joel Yip
Mar 25, 2016

Using the Poyla Form,

n 3 = 6 ( n 3 ) + 6 ( n 2 ) + ( n 1 ) { n }^{ 3 }=6\left( \begin{matrix} n \\ 3 \end{matrix} \right) +6\left( \begin{matrix} n \\ 2 \end{matrix} \right) +\left( \begin{matrix} n \\ 1 \end{matrix} \right)

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 { 2 }^{ n }=\left( \begin{matrix} n \\ 4 \end{matrix} \right) +\left( \begin{matrix} n \\ 3 \end{matrix} \right) +\left( \begin{matrix} n \\ 2 \end{matrix} \right) +\left( \begin{matrix} n \\ 1 \end{matrix} \right) +\left( \begin{matrix} n \\ 0 \end{matrix} \right) \quad for\quad 0\le x\le 4\quad and\quad x\quad is\quad an\quad integer

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 { n }^{ 3 }-{ 2 }^{ n }=-\left( \begin{matrix} n \\ 4 \end{matrix} \right) +5\left( \begin{matrix} n \\ 3 \end{matrix} \right) +5\left( \begin{matrix} n \\ 2 \end{matrix} \right) -\left( \begin{matrix} n \\ 0 \end{matrix} \right) \quad for\quad 0\le x\le 4\quad and\quad x\quad is\quad an\quad integer

0 4 ( x 4 24 + 13 x 3 12 11 x 2 24 7 x 12 1 ) d x = [ x 5 120 + 13 x 4 48 11 x 3 72 7 x 2 24 x ] 0 4 = 1906 45 \int _{ 0 }^{ 4 }{ \left( -\frac { { x }^{ 4 } }{ 24 } +\frac { 13{ x }^{ 3 } }{ 12 } -\frac { 11{ x }^{ 2 } }{ 24 } -\frac { { 7x } }{ 12 } -1 \right) } dx={ \left[ -\frac { { x }^{ 5 } }{ 120 } +\frac { 13{ x }^{ 4 } }{ 48 } -\frac { 11{ x }^{ 3 } }{ 72 } -\frac { 7{ x }^{ 2 } }{ 24 } -x \right] }_{ 0 }^{ 4 }\\ =\frac { 1906 }{ 45 }

1906 45 × 45 = 1906 \\ \frac { 1906 }{ 45 } \times 45=1906

Moderator note:

Good approach.

Or use Boole's Rule :

2 ( 7 f ( 0 ) + 32 f ( 1 ) + 12 f ( 2 ) + 32 f ( 3 ) + 7 f ( 4 ) ) = 2 ( 7 32 + 48 + 608 + 336 ) = 1906 2\left(7f(0)+32f(1)+12f(2)+32f(3)+7f(4)\right)=2(-7-32+48+608+336)=\boxed{1906}

Otto Bretscher - 5 years, 2 months ago

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Also good!

Joel Yip - 5 years, 2 months ago

Don't ignore error term

Joel Yip - 5 years, 2 months ago

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Well, it's a quartic, so that there is no error term; the error term involves the sixth derivative.

Otto Bretscher - 5 years, 2 months ago

Then Boole's rule approximating the integral of f ( x ) 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?

Pi Han Goh - 5 years, 2 months ago

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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.

Otto Bretscher - 5 years, 2 months ago

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@Otto Bretscher Got a proof?

Pi Han Goh - 5 years, 2 months ago

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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.

Otto Bretscher - 5 years, 2 months ago

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@Otto Bretscher Looking forward to it. Thanks!

Pi Han Goh - 5 years, 2 months ago

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@Pi Han Goh There is some good stuff here

Otto Bretscher - 5 years, 2 months ago

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@Otto Bretscher Ohhhh Simpsons rule = specific case of Newton-Cotes Formulas???? I didn't know that! Good to know

Pi Han Goh - 5 years, 2 months ago

Can you write this as a separate solution? It's a very neat one-liner.

Sharky Kesa - 5 years, 2 months ago

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I don't think he got the question correct...

Joel Yip - 5 years, 2 months ago

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.

Otto Bretscher - 5 years, 2 months ago

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@Otto Bretscher Well alright... its not long

Joel Yip - 5 years, 2 months ago

did you get the question correct

Joel Yip - 5 years, 2 months ago

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