Find the positive root of a 4-th degree polynomial equationSolve the equation $x^2+frac9x^2(x+3)^2=27$How many cups of sugar do I need for these 5th grade problems?Ecuation of Parabola from Point in vertex formGrouping and Factoring PolynomialsCoffee Table Trig (Finding angles when working with wider boards)How to solve this equation manually: $(x^2+100)^2=(x^3-100)^3$?Maths Puzzle!!!Largest integer less than $2013$ obtained by repeatedly doubling an integer $x$.Why does the definition of a square root for a number not include its solution's negative counterpart?Determining the number of non-real roots. Multiple choice strategy.Mixed percentage algebra problem

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Find the positive root of a 4-th degree polynomial equation


Solve the equation $x^2+frac9x^2(x+3)^2=27$How many cups of sugar do I need for these 5th grade problems?Ecuation of Parabola from Point in vertex formGrouping and Factoring PolynomialsCoffee Table Trig (Finding angles when working with wider boards)How to solve this equation manually: $(x^2+100)^2=(x^3-100)^3$?Maths Puzzle!!!Largest integer less than $2013$ obtained by repeatedly doubling an integer $x$.Why does the definition of a square root for a number not include its solution's negative counterpart?Determining the number of non-real roots. Multiple choice strategy.Mixed percentage algebra problem













4












$begingroup$


I was struggling with this problem:



$$100^2=x^2+ left( frac100x100+x right)^2$$



It came up when i was developing a solution to a geometry problem. I've already checked in Mathematica and the solution is right, according to the answer. The answer needs to be a real positive number because it's a measure of a segment.



I've tried factoring, manipulating algebraically, but i couldn't solve the resulting 4th degree polynomial. I appreciate if someone could help me. Thanks!










share|cite|improve this question











$endgroup$







  • 1




    $begingroup$
    What's wrong with just using the solution from Mathematica?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    It is a problem from a Olympiad contest. You can't use solvers there.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    math.stackexchange.com/questions/2020139/…
    $endgroup$
    – lab bhattacharjee
    1 hour ago















4












$begingroup$


I was struggling with this problem:



$$100^2=x^2+ left( frac100x100+x right)^2$$



It came up when i was developing a solution to a geometry problem. I've already checked in Mathematica and the solution is right, according to the answer. The answer needs to be a real positive number because it's a measure of a segment.



I've tried factoring, manipulating algebraically, but i couldn't solve the resulting 4th degree polynomial. I appreciate if someone could help me. Thanks!










share|cite|improve this question











$endgroup$







  • 1




    $begingroup$
    What's wrong with just using the solution from Mathematica?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    It is a problem from a Olympiad contest. You can't use solvers there.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    math.stackexchange.com/questions/2020139/…
    $endgroup$
    – lab bhattacharjee
    1 hour ago













4












4








4


1



$begingroup$


I was struggling with this problem:



$$100^2=x^2+ left( frac100x100+x right)^2$$



It came up when i was developing a solution to a geometry problem. I've already checked in Mathematica and the solution is right, according to the answer. The answer needs to be a real positive number because it's a measure of a segment.



I've tried factoring, manipulating algebraically, but i couldn't solve the resulting 4th degree polynomial. I appreciate if someone could help me. Thanks!










share|cite|improve this question











$endgroup$




I was struggling with this problem:



$$100^2=x^2+ left( frac100x100+x right)^2$$



It came up when i was developing a solution to a geometry problem. I've already checked in Mathematica and the solution is right, according to the answer. The answer needs to be a real positive number because it's a measure of a segment.



I've tried factoring, manipulating algebraically, but i couldn't solve the resulting 4th degree polynomial. I appreciate if someone could help me. Thanks!







algebra-precalculus polynomials contest-math real-numbers factoring






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited 3 hours ago









David G. Stork

12.1k41735




12.1k41735










asked 3 hours ago









shewlongshewlong

414




414







  • 1




    $begingroup$
    What's wrong with just using the solution from Mathematica?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    It is a problem from a Olympiad contest. You can't use solvers there.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    math.stackexchange.com/questions/2020139/…
    $endgroup$
    – lab bhattacharjee
    1 hour ago












  • 1




    $begingroup$
    What's wrong with just using the solution from Mathematica?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    It is a problem from a Olympiad contest. You can't use solvers there.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    math.stackexchange.com/questions/2020139/…
    $endgroup$
    – lab bhattacharjee
    1 hour ago







1




1




$begingroup$
What's wrong with just using the solution from Mathematica?
$endgroup$
– David G. Stork
3 hours ago




$begingroup$
What's wrong with just using the solution from Mathematica?
$endgroup$
– David G. Stork
3 hours ago












$begingroup$
It is a problem from a Olympiad contest. You can't use solvers there.
$endgroup$
– shewlong
2 hours ago




$begingroup$
It is a problem from a Olympiad contest. You can't use solvers there.
$endgroup$
– shewlong
2 hours ago












$begingroup$
math.stackexchange.com/questions/2020139/…
$endgroup$
– lab bhattacharjee
1 hour ago




$begingroup$
math.stackexchange.com/questions/2020139/…
$endgroup$
– lab bhattacharjee
1 hour ago










2 Answers
2






active

oldest

votes


















3












$begingroup$

WA tell us that the positive root is
$$
50 (-1 + sqrt 2 + sqrt2 sqrt 2 - 1) approx 88.320
$$

WA also tells us that the minimal polynomial of this number over $mathbb Q$ is
$$
x^4 + 200 x^3 + 10000 x^2 - 2000000 x - 100000000
$$

and so there is no simpler answer.



On the other hand, substituting $x=50u$ in the minimal polynomial gives
$$
6250000 (u^4 + 4 u^3 + 4 u^2 - 16 u - 16)
$$

Now this can factored into two reasonably looking quadratics:
$$
u^4 + 4 u^3 + 4 u^2 - 16 u - 16 =
(u^2 + (2 + 2 sqrt 2) u + 4 sqrt 2 + 4)
(u^2 + (2 - 2 sqrt 2) u - 4 sqrt 2 + 4)
$$

But all this is in hindsight...






share|cite|improve this answer











$endgroup$












  • $begingroup$
    How do we know this is the minimal polynomial? What about $x - 50(-1+sqrt2 + sqrt2sqrt2 -1)= 0$?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    @DavidG.Stork, see my edited answer.
    $endgroup$
    – lhf
    3 hours ago










  • $begingroup$
    OK... I guess we can assume $x in mathbbQ$.
    $endgroup$
    – David G. Stork
    2 hours ago


















0












$begingroup$

Multiplying both sides by $(100+x)^2$, we get
$$
100^2(100+x)^2=x^2(100+x)^2+ 100^2x^2\
Rightarrow100^2[(100+x+x)(100+x-x)]=100^2x^2\
Rightarrow100(100+2x)=x^2\
Rightarrow x^2-200x-10000=0.
$$

You are then able to solve it. The positive root is $100(1+sqrt2)$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Can you explain how you went from step 1 to step 2? I think you've made an error.
    $endgroup$
    – Clayton
    3 hours ago










  • $begingroup$
    I think there's an error in the solution. The answer is not that.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    The right side is (x(100+x))^2.
    $endgroup$
    – shewlong
    2 hours ago











Your Answer





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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









3












$begingroup$

WA tell us that the positive root is
$$
50 (-1 + sqrt 2 + sqrt2 sqrt 2 - 1) approx 88.320
$$

WA also tells us that the minimal polynomial of this number over $mathbb Q$ is
$$
x^4 + 200 x^3 + 10000 x^2 - 2000000 x - 100000000
$$

and so there is no simpler answer.



On the other hand, substituting $x=50u$ in the minimal polynomial gives
$$
6250000 (u^4 + 4 u^3 + 4 u^2 - 16 u - 16)
$$

Now this can factored into two reasonably looking quadratics:
$$
u^4 + 4 u^3 + 4 u^2 - 16 u - 16 =
(u^2 + (2 + 2 sqrt 2) u + 4 sqrt 2 + 4)
(u^2 + (2 - 2 sqrt 2) u - 4 sqrt 2 + 4)
$$

But all this is in hindsight...






share|cite|improve this answer











$endgroup$












  • $begingroup$
    How do we know this is the minimal polynomial? What about $x - 50(-1+sqrt2 + sqrt2sqrt2 -1)= 0$?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    @DavidG.Stork, see my edited answer.
    $endgroup$
    – lhf
    3 hours ago










  • $begingroup$
    OK... I guess we can assume $x in mathbbQ$.
    $endgroup$
    – David G. Stork
    2 hours ago















3












$begingroup$

WA tell us that the positive root is
$$
50 (-1 + sqrt 2 + sqrt2 sqrt 2 - 1) approx 88.320
$$

WA also tells us that the minimal polynomial of this number over $mathbb Q$ is
$$
x^4 + 200 x^3 + 10000 x^2 - 2000000 x - 100000000
$$

and so there is no simpler answer.



On the other hand, substituting $x=50u$ in the minimal polynomial gives
$$
6250000 (u^4 + 4 u^3 + 4 u^2 - 16 u - 16)
$$

Now this can factored into two reasonably looking quadratics:
$$
u^4 + 4 u^3 + 4 u^2 - 16 u - 16 =
(u^2 + (2 + 2 sqrt 2) u + 4 sqrt 2 + 4)
(u^2 + (2 - 2 sqrt 2) u - 4 sqrt 2 + 4)
$$

But all this is in hindsight...






share|cite|improve this answer











$endgroup$












  • $begingroup$
    How do we know this is the minimal polynomial? What about $x - 50(-1+sqrt2 + sqrt2sqrt2 -1)= 0$?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    @DavidG.Stork, see my edited answer.
    $endgroup$
    – lhf
    3 hours ago










  • $begingroup$
    OK... I guess we can assume $x in mathbbQ$.
    $endgroup$
    – David G. Stork
    2 hours ago













3












3








3





$begingroup$

WA tell us that the positive root is
$$
50 (-1 + sqrt 2 + sqrt2 sqrt 2 - 1) approx 88.320
$$

WA also tells us that the minimal polynomial of this number over $mathbb Q$ is
$$
x^4 + 200 x^3 + 10000 x^2 - 2000000 x - 100000000
$$

and so there is no simpler answer.



On the other hand, substituting $x=50u$ in the minimal polynomial gives
$$
6250000 (u^4 + 4 u^3 + 4 u^2 - 16 u - 16)
$$

Now this can factored into two reasonably looking quadratics:
$$
u^4 + 4 u^3 + 4 u^2 - 16 u - 16 =
(u^2 + (2 + 2 sqrt 2) u + 4 sqrt 2 + 4)
(u^2 + (2 - 2 sqrt 2) u - 4 sqrt 2 + 4)
$$

But all this is in hindsight...






share|cite|improve this answer











$endgroup$



WA tell us that the positive root is
$$
50 (-1 + sqrt 2 + sqrt2 sqrt 2 - 1) approx 88.320
$$

WA also tells us that the minimal polynomial of this number over $mathbb Q$ is
$$
x^4 + 200 x^3 + 10000 x^2 - 2000000 x - 100000000
$$

and so there is no simpler answer.



On the other hand, substituting $x=50u$ in the minimal polynomial gives
$$
6250000 (u^4 + 4 u^3 + 4 u^2 - 16 u - 16)
$$

Now this can factored into two reasonably looking quadratics:
$$
u^4 + 4 u^3 + 4 u^2 - 16 u - 16 =
(u^2 + (2 + 2 sqrt 2) u + 4 sqrt 2 + 4)
(u^2 + (2 - 2 sqrt 2) u - 4 sqrt 2 + 4)
$$

But all this is in hindsight...







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited 2 hours ago

























answered 3 hours ago









lhflhf

167k11172404




167k11172404











  • $begingroup$
    How do we know this is the minimal polynomial? What about $x - 50(-1+sqrt2 + sqrt2sqrt2 -1)= 0$?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    @DavidG.Stork, see my edited answer.
    $endgroup$
    – lhf
    3 hours ago










  • $begingroup$
    OK... I guess we can assume $x in mathbbQ$.
    $endgroup$
    – David G. Stork
    2 hours ago
















  • $begingroup$
    How do we know this is the minimal polynomial? What about $x - 50(-1+sqrt2 + sqrt2sqrt2 -1)= 0$?
    $endgroup$
    – David G. Stork
    3 hours ago










  • $begingroup$
    @DavidG.Stork, see my edited answer.
    $endgroup$
    – lhf
    3 hours ago










  • $begingroup$
    OK... I guess we can assume $x in mathbbQ$.
    $endgroup$
    – David G. Stork
    2 hours ago















$begingroup$
How do we know this is the minimal polynomial? What about $x - 50(-1+sqrt2 + sqrt2sqrt2 -1)= 0$?
$endgroup$
– David G. Stork
3 hours ago




$begingroup$
How do we know this is the minimal polynomial? What about $x - 50(-1+sqrt2 + sqrt2sqrt2 -1)= 0$?
$endgroup$
– David G. Stork
3 hours ago












$begingroup$
@DavidG.Stork, see my edited answer.
$endgroup$
– lhf
3 hours ago




$begingroup$
@DavidG.Stork, see my edited answer.
$endgroup$
– lhf
3 hours ago












$begingroup$
OK... I guess we can assume $x in mathbbQ$.
$endgroup$
– David G. Stork
2 hours ago




$begingroup$
OK... I guess we can assume $x in mathbbQ$.
$endgroup$
– David G. Stork
2 hours ago











0












$begingroup$

Multiplying both sides by $(100+x)^2$, we get
$$
100^2(100+x)^2=x^2(100+x)^2+ 100^2x^2\
Rightarrow100^2[(100+x+x)(100+x-x)]=100^2x^2\
Rightarrow100(100+2x)=x^2\
Rightarrow x^2-200x-10000=0.
$$

You are then able to solve it. The positive root is $100(1+sqrt2)$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Can you explain how you went from step 1 to step 2? I think you've made an error.
    $endgroup$
    – Clayton
    3 hours ago










  • $begingroup$
    I think there's an error in the solution. The answer is not that.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    The right side is (x(100+x))^2.
    $endgroup$
    – shewlong
    2 hours ago















0












$begingroup$

Multiplying both sides by $(100+x)^2$, we get
$$
100^2(100+x)^2=x^2(100+x)^2+ 100^2x^2\
Rightarrow100^2[(100+x+x)(100+x-x)]=100^2x^2\
Rightarrow100(100+2x)=x^2\
Rightarrow x^2-200x-10000=0.
$$

You are then able to solve it. The positive root is $100(1+sqrt2)$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Can you explain how you went from step 1 to step 2? I think you've made an error.
    $endgroup$
    – Clayton
    3 hours ago










  • $begingroup$
    I think there's an error in the solution. The answer is not that.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    The right side is (x(100+x))^2.
    $endgroup$
    – shewlong
    2 hours ago













0












0








0





$begingroup$

Multiplying both sides by $(100+x)^2$, we get
$$
100^2(100+x)^2=x^2(100+x)^2+ 100^2x^2\
Rightarrow100^2[(100+x+x)(100+x-x)]=100^2x^2\
Rightarrow100(100+2x)=x^2\
Rightarrow x^2-200x-10000=0.
$$

You are then able to solve it. The positive root is $100(1+sqrt2)$.






share|cite|improve this answer









$endgroup$



Multiplying both sides by $(100+x)^2$, we get
$$
100^2(100+x)^2=x^2(100+x)^2+ 100^2x^2\
Rightarrow100^2[(100+x+x)(100+x-x)]=100^2x^2\
Rightarrow100(100+2x)=x^2\
Rightarrow x^2-200x-10000=0.
$$

You are then able to solve it. The positive root is $100(1+sqrt2)$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered 3 hours ago









Holding ArthurHolding Arthur

1,555417




1,555417











  • $begingroup$
    Can you explain how you went from step 1 to step 2? I think you've made an error.
    $endgroup$
    – Clayton
    3 hours ago










  • $begingroup$
    I think there's an error in the solution. The answer is not that.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    The right side is (x(100+x))^2.
    $endgroup$
    – shewlong
    2 hours ago
















  • $begingroup$
    Can you explain how you went from step 1 to step 2? I think you've made an error.
    $endgroup$
    – Clayton
    3 hours ago










  • $begingroup$
    I think there's an error in the solution. The answer is not that.
    $endgroup$
    – shewlong
    2 hours ago










  • $begingroup$
    The right side is (x(100+x))^2.
    $endgroup$
    – shewlong
    2 hours ago















$begingroup$
Can you explain how you went from step 1 to step 2? I think you've made an error.
$endgroup$
– Clayton
3 hours ago




$begingroup$
Can you explain how you went from step 1 to step 2? I think you've made an error.
$endgroup$
– Clayton
3 hours ago












$begingroup$
I think there's an error in the solution. The answer is not that.
$endgroup$
– shewlong
2 hours ago




$begingroup$
I think there's an error in the solution. The answer is not that.
$endgroup$
– shewlong
2 hours ago












$begingroup$
The right side is (x(100+x))^2.
$endgroup$
– shewlong
2 hours ago




$begingroup$
The right side is (x(100+x))^2.
$endgroup$
– shewlong
2 hours ago

















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