Math Is Fun Forum
  Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

#1 2008-07-01 23:51:02

Registered: 2007-08-03
Posts: 192

Solve in positive integers the cubic

Solve in positive integers the cubic



#2 2008-07-02 02:07:19

Real Member
Registered: 2008-06-08
Posts: 1,889

Re: Solve in positive integers the cubic

The solution (obviously) is 13 but you know how Dirty n Untidy n Ugly the cubic equations are to solve!!
If anyone's got a Good way to solve the above, please let me know!!:cool:

If two or more thoughts intersect, there has to be a point!


#3 2011-07-28 22:43:45

Registered: 2011-01-09
Posts: 3,481

Re: Solve in positive integers the cubic


"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."


#4 2011-08-02 19:34:04

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

Re: Solve in positive integers the cubic


He probably graphed the equation and eyeballed the root as 13. This is an acceptable conjecture which can be proved by plugging in. A little fortuitious but... Then it is trival to prove that it is the only positive root.

You would deflate out the obvious root.

The discriminant of the right side is - 472, so 13 is the only positive root.

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.


#5 2012-05-02 11:35:35

Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

Re: Solve in positive integers the cubic

Another way is to use the method developed for solving a general cubic. It is given in a thread of mine and is available on the net.

“Here lies the reader who will never open this book. He is forever dead.
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.


Board footer

Powered by FluxBB