Math Is Fun Forum

Rose-Red

Member

Registered: 2006-01-05

Posts: 19

weird equation

Could someone help me with this?

x³=6x+9

It doesn't seem difficult, but my answer doesn't suit to the answer in the book

God

Member

Registered: 2005-08-25

Posts: 59

Re: weird equation

so basically x^3 - 6x - 9 = 0

We can factor out (x-3) from here, leaving
(x-3)(x^2+3*x+3) = 0

x-3 = 0, so x=3 is a solution

x^2 + 3x + 3 = 0
This can be solved easily using the quadratic formula

Rose-Red

Member

Registered: 2006-01-05

Posts: 19

Re: weird equation

thanx man, but I did it in another more complicated way.

x³-9-6x=0
(x³-27)+18-6x=0
(x-3)(x²+27x+27²)-6(x-3)=0
(x-3)(x²+27x+723)=0

x-3=0
x=3

and
x²+27x+723=0
D<0 no solutions

Now the answer is correct.

Could u explain me how you get this (x^2+3*x+3)?

Last edited by Rose-Red (2006-01-08 09:21:57)

irspow

Member

Registered: 2005-11-24

Posts: 1,055

Re: weird equation

God factored out x-3 from the original equation.  (x-3)(x²+3x+3) = x³-6x-9

---

I am at an age where I have forgotten more than I remember, but I still pretend to know it all.

Rose-Red

Member

Registered: 2006-01-05

Posts: 19

Re: weird equation

but i dont understand how it is possible. I cant find any rule or formula that allows to do that

God

Member

Registered: 2005-08-25

Posts: 59

Re: weird equation

By the rational roots theorem, the only possible rational solutions are -9 -3 -1 1 3 and 9
By the rule of signs, there is one sign change, so there is a positive root, so start with positive numbers.
After finding that 3 is a root, we know that we can factor out (x-3)

Use synthetic division to find out the remaining quadratic:

Rose-Red

Member

Registered: 2006-01-05

Posts: 19

Re: weird equation

OMG I've never hear of that O_O
Thank you, I'll try to understand that and maybe use it later