Math Is Fun Forum

Agnishom

Real Member

From: Riemann Sphere

Registered: 2011-01-29

Posts: 24,974

Website

What values of x make this expression a perfect cube?

What values of x make this expression a perfect cube?

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'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: What values of x make this expression a perfect cube?

Hi Agnishom;

Any restrictions on the x?

Did you look at the sum question you posted?

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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.

Agnishom

Real Member

From: Riemann Sphere

Registered: 2011-01-29

Posts: 24,974

Website

Re: What values of x make this expression a perfect cube?

x is an Integer. I am getting -4. But that is not working.

Link Please?

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'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: What values of x make this expression a perfect cube?

x = - 5 is also an answer.

For the link:

http://www.mathisfunforum.com/viewtopic … 18#p319718

---

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.

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

Re: What values of x make this expression a perfect cube?

x = - 5 is also an answer.

-216 is not a perfect square.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: What values of x make this expression a perfect cube?

That is true but he wants cubes.

---

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.

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

Re: What values of x make this expression a perfect cube?

I think I am going blind.

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

Re: What values of x make this expression a perfect cube?

x is an Integer. I am getting -4. But that is not working.

I think -4 does work, unless I am not reading properly again.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: What values of x make this expression a perfect cube?

It is a solution but it is probably not working because there are at least 2 solutions and he is entering only 1.

---

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.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,131

Re: What values of x make this expression a perfect cube?

I'm getting -5, -4, -3, and -2. (so far)

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Agnishom

Real Member

From: Riemann Sphere

Registered: 2011-01-29

Posts: 24,974

Website

Re: What values of x make this expression a perfect cube?

Visit an optician maybe?

How are you doing this?

---

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

Re: What values of x make this expression a perfect cube?

T&E, I hope.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,131

Re: What values of x make this expression a perfect cube?

Do you agree with those answers first ?

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: What values of x make this expression a perfect cube?

Hi Bob

I am getting those answers, too.

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“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.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,131

Re: What values of x make this expression a perfect cube?

Thanks.  I have to confess I don't have a method that proves those are the only solutions.  I just used Excel to slog through a large set of possibles. 

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: What values of x make this expression a perfect cube?

I do have a method. I will post it here. As Bob would always say, watch this space.

Solution

Last edited by anonimnystefy (2014-06-10 05:21:17)

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“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.

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

Re: What values of x make this expression a perfect cube?

Thanks.  I have to confess I don't have a method that proves those are the only solutions.  I just used Excel to slog through a large set of possibles. 

Bob

In the US we call that cheating.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: What values of x make this expression a perfect cube?

Depends on where in the US. Some call it EM.

---

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.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: What values of x make this expression a perfect cube?

Done. There might be mistakes though, so there may be more solutions.

---

“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.

ShivamS

Member

Registered: 2011-02-07

Posts: 3,648

Re: What values of x make this expression a perfect cube?

Now I know Stefy's MTG username.

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: What values of x make this expression a perfect cube?

MTG? Magic: The Gathering?

---

“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.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: What values of x make this expression a perfect cube?

Your solution?

---

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.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,131

Re: What values of x make this expression a perfect cube?

But it is leading me to a 'proper' solution.

I got to (2x+7)^3 + 63(2x+7) and then Stefy posted.  So I'll give the glory to him. 

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

anonimnystefy

Real Member

From: Harlan's World

Registered: 2011-05-23

Posts: 16,049

Re: What values of x make this expression a perfect cube?

I have no idea who is talking to whom anymore.

Thanks, Bob. I am pretty sure that you would've got it if you continued.

Last edited by anonimnystefy (2014-06-10 05:37:43)

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“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.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,131

Re: What values of x make this expression a perfect cube?

hi Stefy,

Your solution looks good to me.  I think you've proved those are the only solutions. 

Bob

ps.  My excel went from x = -10 to + 50 and I noticed that all the cube roots are symmetrically placed around x = -3.5.  So its a good demonstration of bobbym's mantra:

get some data first.

Last edited by Bob (2014-06-10 05:40:19)

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob