MHB How Can You Simplify This Complex Fraction Problem?

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The discussion focuses on simplifying the complex fraction problem involving the expression $\frac{1^4 + 2007^4 + 2008^4}{1^2 + 2007^2 + 2008^2}$. Various methods are explored, including expansion using the Binomial Theorem and modular arithmetic to derive congruences. The conclusion reached is that the value of the fraction simplifies to 4,030,057. Participants also discuss alternative approaches, such as factorization and polynomial simplification, to verify the solution. Overall, the thread emphasizes the effectiveness of different mathematical strategies in solving the problem.
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Problem:

Find the value of $\displaystyle \frac{1^4+2007^4+2008^4}{1^2+2007^2+2008^2}$.

My Attempt:

For the numerator, I expand them using the Binomial Theorem and get:

$\displaystyle 1^4+2007^4+2008^4$

$\displaystyle=1+(2000+7)^4+(2000+8)^4$

$\displaystyle
=1+2000^4+4(2000)^3(7)^1+6(2000)^2(7)^2+4(2000)^1(7)^3+7^4$

$\displaystyle
+2000^4+4(2000)^3(8)^1+6(2000)^2(8)^2+4(2000)^1(8)^3+8^4$

$\displaystyle
=2(2000^4)+60(2000)^3+678(2000)^2+3420(2000)+6498$I do the same for the denominator and get:

$\displaystyle 1^2+2007^2+2008^2$

$\displaystyle=1+(2000+7)^2+(2000+8)^2$

$\displaystyle=1+(2000)^2+14(2000)+49+(2000)^2+16(2000)+64$

$\displaystyle=2(2000^2)+30(2000)+114$
I noticed that if I square the denominator, I get twice the value as the numerator, let's see...

$\displaystyle (1^2+2007^2+2008^2)^2$

$\displaystyle
=4(2000^4)+120(2000)^3+1356(2000)^2+6840(2000)+12996$

$\displaystyle
=2(2(2000^4)+60(2000)^3+678(2000)^2+3420(2000)+6498)$So, we can say that
$\displaystyle \frac{1^4+2007^4+2008^4}{(1^2+2007^2+2008^2)^2}= \frac{1}{2}$

Therefore,
$\displaystyle \frac{1^4+2007^4+2008^4}{1^2+2007^2+2008^2}=\frac{1}{2}(1^2+2007^2+2008^2)=\frac{1}{2} (2(2000^2)+30(2000)+114)=4,030,057$My question is, are there any other approaches to solve this problem?

Thanks in advance.
 
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You can use the well known relations...

$\displaystyle \sum_{k=1}^{n} k^{2} = \frac{n\ (n+1)}{2}\ \frac{2 n +1}{3}$ (1)

$\displaystyle \sum_{k=1}^{n} k^{4} = \frac{n\ (n+1)}{2}\ \frac{2 n +1}{3}\ \frac{3 n^{2} + 3 n -1}{5} $ (2)

... but I doubt that that is a more comfortable way respect to direct computation...

Kind regards

$\chi$ $\sigma$
 
If you know in advance that the fraction is an integer, you can use the following approach.

--------​

Let $f$ be the fraction we want to evaluate. Select $n = 2007$. Then:

$$1^4 + 2007^4 + 2008^4 \equiv 1^4 + 0^4 + 1^4 \equiv 2 \pmod{2007}$$
$$1^2 + 2007^2 + 2008^2 \equiv 1^2 + 0^2 + 1^2 \equiv 2 \pmod{2007}$$
Therefore:

$$f \equiv 1 \pmod{2007} \tag{1}$$
Now use $n = 2008$. We get:

$$1^4 + 2007^4 + 2008^4 \equiv 1^4 + (-1)^4 + 0^4 \equiv 2 \pmod{2008}$$
$$1^2 + 2007^2 + 2008^2 \equiv 1^2 + (-1)^2 + 0^2 \equiv 2 \pmod{2008}$$
And therefore:

$$f \equiv 1 \pmod{2008} \tag{2}$$
And we note that $\gcd{(2007, 2008)} = 1$, and so, using the CRT on (1) and (2):

$$f \equiv 1 \pmod{2007 \times 2008} ~ ~ \implies ~ ~ f \equiv 1 \pmod{4030056} \tag{3}$$
And now, observe that we can write:

$$\frac{1^4 + 2007^4 + 2008^4}{1^2 + 2007^2 + 2008^2} \approx \frac{2 \cdot 2000^4}{2 \cdot 2000^2} \approx 2000^2 \approx 4000000 \tag{4}$$
Which shows that the correct solution must be $f = 4030057$. To be absolutely rigorous, it is enough to prove that:

$$\text{The approximation error is less than the distance between two possible solutions}$$
$$\therefore$$
$$\left | \frac{1^4 + 2007^4 + 2008^4}{1^2 + 2007^2 + 2008^2} - \frac{2 \cdot 2000^4}{2 \cdot 2000^2} \right | < 4030056$$
--------​

This probabilistic approach may be overkill for the problem and is perhaps not what you were looking for, but I thought this was worth posting to illustrate a different -albeit number-theoretical - point of view.
 
Last edited:
Bacterius said:
If you know in advance that the fraction is an integer, you can use the following approach.

--------​

Let $f$ be the fraction we want to evaluate. Select $n = 2007$. Then:

$$1^4 + 2007^4 + 2008^4 \equiv 1^4 + 0^4 + 1^4 \equiv 2 \pmod{2007}$$
$$1^2 + 2007^2 + 2008^2 \equiv 1^2 + 0^2 + 1^2 \equiv 2 \pmod{2007}$$
Therefore:

$$f \equiv 1 \pmod{2007} \tag{1}$$
Now use $n = 2008$. We get:

$$1^4 + 2007^4 + 2008^4 \equiv 1^4 + (-1)^4 + 0^4 \equiv 2 \pmod{2008}$$
$$1^2 + 2007^2 + 2008^2 \equiv 1^2 + (-1)^2 + 0^2 \equiv 2 \pmod{2008}$$
And therefore:

$$f \equiv 1 \pmod{2008} \tag{2}$$
And we note that $\gcd{(2007, 2008)} = 1$, and so, using the CRT on (1) and (2):

$$f \equiv 1 \pmod{2007 \times 2008} ~ ~ \implies ~ ~ f \equiv 1 \pmod{4030056} \tag{3}$$
And now, observe that we can write:

$$\frac{1^4 + 2007^4 + 2008^4}{1^2 + 2007^2 + 2008^2} \approx \frac{2 \cdot 2000^4}{2 \cdot 2000^2} \approx 2000^2 \approx 4000000 \tag{4}$$
Which shows that the correct solution must be $f = 4030057$, as the approximation is more than sufficient:

$$\left | \frac{1^4 + 2007^4 + 2008^4}{1^2 + 2007^2 + 2008^2} - \frac{2 \cdot 2000^4}{2 \cdot 2000^2} \right | < 4030056$$
--------​

This probabilistic approach may be overkill for the problem and is perhaps not what you were looking for, but I thought this was worth posting to illustrate a different -albeit number-theoretical - point of view.
Thanks, Bacterius, as you may have already noticed, it's all Greek to me.:o But I enjoy reading it very much...(Happy)
 
chisigma said:
You can use the well known relations...

$\displaystyle \sum_{k=1}^{n} k^{2} = \frac{n\ (n+1)}{2}\ \frac{2 n +1}{3}$ (1)

$\displaystyle \sum_{k=1}^{n} k^{4} = \frac{n\ (n+1)}{2}\ \frac{2 n +1}{3}\ \frac{3 n^{2} + 3 n -1}{5} $ (2)

... but I doubt that that is a more comfortable way respect to direct computation...

Kind regards

$\chi$ $\sigma$

Thanks, chisigma...what you've suggested has certainly given me food for thought, but I've got to run now, I will work with it to check if your suggestion could be simplified out neatly and that we don't have to deal with bigger figures too.
 
anemone said:
Problem:

Find the value of $\displaystyle \frac{1^4+2007^4+2008^4}{1^2+2007^2+2008^2}$.

My Attempt:

For the numerator, I expand them using the Binomial Theorem and get:

$\displaystyle 1^4+2007^4+2008^4$

$\displaystyle=1+(2000+7)^4+(2000+8)^4$

$\displaystyle
=1+2000^4+4(2000)^3(7)^1+6(2000)^2(7)^2+4(2000)^1(7)^3+7^4$

$\displaystyle
+2000^4+4(2000)^3(8)^1+6(2000)^2(8)^2+4(2000)^1(8)^3+8^4$

$\displaystyle
=2(2000^4)+60(2000)^3+678(2000)^2+3420(2000)+6498$I do the same for the denominator and get:

$\displaystyle 1^2+2007^2+2008^2$

$\displaystyle=1+(2000+7)^2+(2000+8)^2$

$\displaystyle=1+(2000)^2+14(2000)+49+(2000)^2+16(2000)+64$

$\displaystyle=2(2000^2)+30(2000)+114$
I noticed that if I square the denominator, I get twice the value as the numerator, let's see...

$\displaystyle (1^2+2007^2+2008^2)^2$

$\displaystyle
=4(2000^4)+120(2000)^3+1356(2000)^2+6840(2000)+12996$

$\displaystyle
=2(2(2000^4)+60(2000)^3+678(2000)^2+3420(2000)+6498)$So, we can say that
$\displaystyle \frac{1^4+2007^4+2008^4}{(1^2+2007^2+2008^2)^2}= \frac{1}{2}$

Therefore,
$\displaystyle \frac{1^4+2007^4+2008^4}{1^2+2007^2+2008^2}=\frac{1}{2}(1^2+2007^2+2008^2)=\frac{1}{2} (2(2000^2)+30(2000)+114)=4,030,057$My question is, are there any other approaches to solve this problem?

Thanks in advance.
I think that the OP's solution is as good as you can get. My approach was to calculate $1+x^4+(x+1)^4 = 2(x^4+2x^3+3x^2 + 2x+1) = 2(x^2+x+1)^2$, and $1+x^2+(x+1)^2 = 2(x^2+x+1)$, so that $$\frac{1+x^4+(x+1)^4}{1+x^2+(x+1)^2} = \frac{2(x^2+x+1)^2}{2(x^2+x+1)} = x^2+x+1.$$ That is the same as what anemone did for $x=2007$. Then when you use the fact that $2007 = 2000 + 7$, you easily get the answer $4030057$.
 
Opalg said:
I think that the OP's solution is as good as you can get. My approach was to calculate $1+x^4+(x+1)^4 = 2(x^4+2x^3+3x^2 + 2x+1) = 2(x^2+x+1)^2$, and $1+x^2+(x+1)^2 = 2(x^2+x+1)$, so that $$\frac{1+x^4+(x+1)^4}{1+x^2+(x+1)^2} = \frac{2(x^2+x+1)^2}{2(x^2+x+1)} = x^2+x+1.$$ That is the same as what anemone did for $x=2007$. Then when you use the fact that $2007 = 2000 + 7$, you easily get the answer $4030057$.

I was trying to post this , but the browser just crashed ... :)
 
Opalg said:
I think that the OP's solution is as good as you can get. My approach was to calculate $1+x^4+(x+1)^4 = 2(x^4+2x^3+3x^2 + 2x+1) = 2(x^2+x+1)^2$, and $1+x^2+(x+1)^2 = 2(x^2+x+1)$, so that $$\frac{1+x^4+(x+1)^4}{1+x^2+(x+1)^2} = \frac{2(x^2+x+1)^2}{2(x^2+x+1)} = x^2+x+1.$$ That is the same as what anemone did for $x=2007$. Then when you use the fact that $2007 = 2000 + 7$, you easily get the answer $4030057$.

Hi Opalg, your approach is much more better than mine because I didn't think of $2(x^4+2x^3+3x^2 + 2x+1)$ could be factorized as $ 2(x^2+x+1)^2$, thanks for giving me a lot of inspirations (both in the past and now) on how to tackle a maths problem more effectively.

Thanks!:)
 
Hello, anemone

I have a different approach.
I wouldn't claim that it is "better".

$\text{Evaluate: }\:N \;=\;\dfrac{2008^4+2007^4 + 1^4}{2008^2 + 2007^2+1^2}$
Let $u \:=\:2008$

We have: .$N \;=\;\dfrac{u^4 + (u-1)^4 + 1}{u^2 + (u-1)^2 + 1} \;=\;\dfrac{u^4+u^4-4u^3+6u^2-4u+1+1}{u^2+u^2-2u+1+1} $

. . . . . . . . . .$=\; \dfrac{2u^4-4u^3 + 6u^2 - 4u + 2}{2u^2-2u+2} \;=\;\dfrac{2(u^4-2u^3+3u^2 - 2u + 1)}{2(u^2-u+1)} $

. . . . . . . . . .$=\;\dfrac{u^4-2u^3 + 3u^2 - 2u + 1}{u^2-u+1} \;=\;\dfrac{(u^2-u+1)(u^2-u+1)}{u^2-u+1} $

. . . . . . . . . .$=\;u^2-u+1$Back-substitute: .$N \;=\;2008^2 - 2008 + 1 \;=\;4,\!030,\!057$
 
  • #10
soroban said:
$=\;\dfrac{u^4-2u^3 + 3u^2 - 2u + 1}{u^2-u+1} \;=\;\dfrac{(u^2-u+1)(u^2-u+1)}{u^2-u+1} $

. . . . . . . . . .$=\;u^2-u+1$

Just in case factorizing isn't clear , long division will do the task ...
 
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