Cubic equation

Cubic equation

Postby Guest » Wed May 13, 2015 5:59 pm

actually, not sure if it belongs into this group:

I have the following problem with the cubic equation:
20x^3+8^2+7x+6=0

Is there some kind of cubic equation (just as the quadratic one)? According to wikipedia there, is:

http://en.wikipedia.org/wiki/Cubic_function
(under the title: "critical points of a cubic function")

So I tried to apply it just like the quadratic function. Sadly, my root-funtion turns out to be negative:

the square root of: 8^2-3*20*7 --> negative, so not possible

what am I doing wrong??
would really appreciate if sbdy could help!!! :?: :?:
Guest
 

Re: Cubic equation

Postby Guest » Wed May 13, 2015 7:27 pm

The section of wikipedia you are referring to calculates the turning points (also called stationary points) of the cubic, not the roots.
Because the determinant was negative there are no turning points for your equation, it just continually increases.

If you want to calculate the roots, you need to look at the section titled "general formula for roots".
http://en.wikipedia.org/wiki/Cubic_func ... _for_roots
Because there are no turning points there will be exactly one real root (and two complex roots).
Choose k=1 to get the real root. The formula you get at the end is horrible, you may want to use the below link instead.

https://www.wolframalpha.com/input/?i=20x^3%2B8x^2%2B7x%2B6%3D0
calculates the roots for you (click the "exact form" button in the "real solution" section).

Hope this helped,

R. Baber.
Guest
 

Re: Cubic equation

Postby HallsofIvy » Thu Dec 19, 2019 12:20 pm

Yes, there is a "cubic formula" one form of it is given at
https://math.vanderbilt.edu/schectex/courses/cubic/

HallsofIvy
 
Posts: 145
Joined: Sat Mar 02, 2019 9:45 am
Reputation: 58


Return to Polynomials, Polynomial Identities



Who is online

Users browsing this forum: No registered users and 4 guests