- #1
Gabriel Maia
- 72
- 1
Hi everyone. I'm sorry for the long thread. If you don't want to read all the introductory stuff I will mark the part towards the end where my questions are located. I'm trying to find the general formulae for the roots of the equation
$$ax^3 + bx^2 + cx + d = 0$$
By using some changes of variable (which does not really matter now) I was able to rewrite this equation as
$$z^3 - \frac{\Delta_{_{0}}}{3a^2}z+\frac{\Delta_{_{1}}}{27a^3} = 0$$
I used then the Vieta's substitution
$$z = w + \frac{\Delta_{_{0}}}{9a^2w}$$
to obtain the following:
$$(w^3)^2 + \frac{\Delta_{_{1}}}{27a^3}w^3 + \frac{\Delta_{_{0}}^{3}}{729a^6} = 0$$
This is a quadratic equation on [itex]w^3[/itex] with roots:
$$ w^3 = -\frac{1}{27a^3}\,\left(\frac{\Delta_{_{1}} \mp \sqrt{\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3}}}{2}\right)$$
Now, if [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} = 0[/itex], then [itex]w[/itex] has three possible values for each sign and all these six values are the same.
If [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} > 0[/itex] there are three distinct values for each sign in [itex]w[/itex], making a total of six distinct roots.
Finally, if [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} < 0[/itex] we will have, I think, three different complex roots for each sign in [itex]w[/itex], right?
[itex]{\Large AND \,\,\, NOW, \,\,\, FOR \,\,\, THE \,\,\, REAL \,\,\, QUESTIONS:}[/itex]
- In the case where [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} > 0[/itex] there is one obvious root, but how can I find the others?
- In the case where [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} < 0[/itex] there is, again, one obvious root, but how can I find the others? And more... how can I be sure they're all different from each other?- If there are at least two distinct roots for each possible sign of [itex]w[/itex] we have a total of four distinct values. But since each value of [itex]w[/itex] gives one value of [itex]z[/itex], because they are related by
$$z = w + \frac{\Delta_{_{0}}}{9a^2w},$$
I would have four distinct values of [itex]z[/itex] but it is the root of a cubic equation so there should be at most three distinct values. What is going on here? Thank you very much.
$$ax^3 + bx^2 + cx + d = 0$$
By using some changes of variable (which does not really matter now) I was able to rewrite this equation as
$$z^3 - \frac{\Delta_{_{0}}}{3a^2}z+\frac{\Delta_{_{1}}}{27a^3} = 0$$
I used then the Vieta's substitution
$$z = w + \frac{\Delta_{_{0}}}{9a^2w}$$
to obtain the following:
$$(w^3)^2 + \frac{\Delta_{_{1}}}{27a^3}w^3 + \frac{\Delta_{_{0}}^{3}}{729a^6} = 0$$
This is a quadratic equation on [itex]w^3[/itex] with roots:
$$ w^3 = -\frac{1}{27a^3}\,\left(\frac{\Delta_{_{1}} \mp \sqrt{\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3}}}{2}\right)$$
Now, if [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} = 0[/itex], then [itex]w[/itex] has three possible values for each sign and all these six values are the same.
If [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} > 0[/itex] there are three distinct values for each sign in [itex]w[/itex], making a total of six distinct roots.
Finally, if [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} < 0[/itex] we will have, I think, three different complex roots for each sign in [itex]w[/itex], right?
[itex]{\Large AND \,\,\, NOW, \,\,\, FOR \,\,\, THE \,\,\, REAL \,\,\, QUESTIONS:}[/itex]
- In the case where [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} > 0[/itex] there is one obvious root, but how can I find the others?
- In the case where [itex]\Delta_{_{1}}^{2}-4\Delta_{_{0}}^{3} < 0[/itex] there is, again, one obvious root, but how can I find the others? And more... how can I be sure they're all different from each other?- If there are at least two distinct roots for each possible sign of [itex]w[/itex] we have a total of four distinct values. But since each value of [itex]w[/itex] gives one value of [itex]z[/itex], because they are related by
$$z = w + \frac{\Delta_{_{0}}}{9a^2w},$$
I would have four distinct values of [itex]z[/itex] but it is the root of a cubic equation so there should be at most three distinct values. What is going on here? Thank you very much.