Math Is Fun Forum

manich44

Member

Registered: 2009-11-29

Posts: 14

Cubic Formula Example

Hi,

Can someone show me how to solve this cubic equation

h3 + 87.48h - 305.73 = 0

Change the figures please so I can work it out myself,just want a step by step example

Thanks Mark

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Cubic Formula Example

Hi Mark;

h3 + 87.48h - 305.73 = 0

I hope you mean:

---

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.

manich44

Member

Registered: 2009-11-29

Posts: 14

Re: Cubic Formula Example

Hi bobbym,

Yes thats what i mean.I know h = 3.14 because Ive done it on a calculator but could you show me how to get there step by step.

Thanks...Mark

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Cubic Formula Example

Hi Mark;

Please go here to see everything that can be done algebraically to solve that type of equation.

http://en.wikipedia.org/wiki/Cubic_function

Mind you in the real world no one ever uses any of those methods. We use iterative techniques so that the roots can be found by numerical methods.

---

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.

manich44

Member

Registered: 2009-11-29

Posts: 14

Re: Cubic Formula Example

Hi,
Is iteration where you create a table of values and work it out that way?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Cubic Formula Example

Hi Manich;

The simplest method is interval bisection but all these methods unlike the methods on that page are best done by computer.

---

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.

manich44

Member

Registered: 2009-11-29

Posts: 14

Re: Cubic Formula Example

Got to
p3 + q4
= 24794.911296 + 23367.708223
=48162.61952

How  do I key cubrt 48162.619521 - 152.865  + cubrt -48162.619521-152.865 ito a calculator?

Thanks...

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Cubic Formula Example

Hi

Is this what you are trying to do?

cubrt (48162.619521 - 152.865)  + cubrt (-48162.619521-152.865)

or

cubrt (48162.619521) - 152.865  + cubrt (-48162.619521) -152.865

---

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.

manich44

Member

Registered: 2009-11-29

Posts: 14

Re: Cubic Formula Example

The first one
Thanks..

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Cubic Formula Example

Hi;

That is why I do not use those methods. All calculators should have a cube root key. If not do the parentheses first and raise each parentheses to the .3333333333 power. That should be close enough.

---

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.