Re: Solving a cubic equation using trigonometry, but with a2 term? Message #2 Posted by Dieter on 29 Sept 2011, 7:19 a.m., in response to message #1 by snaggs
Sure there is no a2 term: That's why it says "A general cubic equation can be transformed to the following form using a substitution". ;) For more details see below.
There are wellknown standard methods for solving cubic equations. Just take a look at de.wikipedia.org  as well as the last link ("Leitfaden") on that page for a stepbystep solution. The entry on
en.wikipedia.org is has some more details on the numerics.
Having read all this you may finally proceed to the actual algorithm. ;) For instance by using Cardano's method as described on de.wikipedia.org. This is the method you will probably use.
Edit 1:
Oops, I was a bit too fast. ;) I didn't notice you're not Thomas so you probably don't read German. However, the mentioned Cardano method first reduces the cubic equation to a simplified form without "a2 term" and then this reduced equation is solved. The latter part is already done by the program you got. So you just have to provide the initial reduction (or transformation, as Thomas said) the way it is described in the wikipedia articles. The rest is already there.
Edit 2:
All this has already been discussed in the thread you mentioned. Please read messages 3  5 there. ;)
Dieter
Edited: 29 Sept 2011, 8:49 a.m.
