Question

how do i find the zeros of f(x)=x^3-3x^2-3?

Answer 1

find a zero by hit and trial method. Next u get a factor so divide by it and then find the rest of the zeroes

Answer 2

hum i now this!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Answer 3

this can not be factored!
http://www.wolframalpha.com/input/?i=x%5E3+-+3x%5E2+-+3+%3D+0

Answer 4

@RadEn indeed this is not reducible over the reals... check the discriminant.

Answer 5

actually the discriminant used in quadratic equation, not for cubic eq

Answer 6

Correct @RadEn

Answer 7

@RadEn the discriminant you are likely familiar with is \(\Delta=b^2-4ac\), and it is (indeed) only for quadratic equations \(ax^2+bx+c=0\). But the discriminant as a concept is extended to polynomials of other degree...
http://en.wikipedia.org/wiki/Discriminant
For example, a cubic equation \(ax^3+bx^2+cx+d=0\) yields a discriminant \(b^2c^2-4ac^3-4b^3d-27a^2d^2+18abcd\), a homogeneous polynomial of degree 4.

Answer 8

http://en.wikipedia.org/wiki/Cubic_polynomial#The_nature_of_the_roots
So let us check our discriminant now :-)$$\Delta=b^2c^2-4ac^3-4b^3d-27a^2d^2+18abcd$$Here, we have \(x^3-3x^2-3=0\) so \(a=1, b=-3,c=0,d=-3\). Plug them in:$$\begin{align*}\Delta&=27(1)^2(-3)^2-4(1)(0)-4(-3)^3(-3)-27(1)^2(-3)^2+18(1)(-3)(0)(-3)\\&=27\times9-0-4\times81-27\times9+0\\&=-4\times81\lt0\end{align*}$$Because \(\Delta\lt0\), we have only one real root and two complex conjugate roots.

Answer 9

No need of such big discriminants coz one can always use hit and trial method to find the zero of a simple equation like this..... and then use the discriminant for the quadratic equation after creating a factor with that zero.

Answer 10

@lordcyborg the real root is not trivial here, so tell me more about how you plan on using a simple "hit and trial" method :-) In fact, this polynomial's only real root isn't rational.

Answer 11

hmmmm @oldrin.bataku I did not take a good look at d equation. So I suggested to use just hit and trial method. Yup u r correct. Sorry for arguing!!!!!!

Answer 12

@lordcyborg no argument here! Just discussing math :-p