How to return the original polynomial from the calculated roots

Hi MATLAB experts,

I have following example:

Solve the equation 3x^2-2x-4

Create a vector to represent the polynomial, then find the roots.

>> p=[3 -2 -4];

>> r=roots(p)

ans =

1.5352

-0.8685

I find the roots and wanted to return back the same polynomial coefficients i used the follwing code:

>> poly(r)

ans =

1.0000 -0.6667 -1.3333

But the coefficients are not the same as the original polynomial coefficients.

The question is how to get or return the original polynomial coefficients?

Thanks & regards,

Basim Touqan

Answer 1

[Dyuman Joshi]

So, the coefficients are not the same, but they are similar.

%[1 -0.6667 -1.3333] is equal to [3 -2 4]/3.

This is because, poly() calculates equation in the form of x^2+b*x+c (a=1) instead of the generalised form a*x^2+b*x+c (i.e. the function is written in such a way).

If you want your solution to be in the generalised form, what you can do is multiply the equation with the first member of original equation.

p=[3 -2 4];
r=roots(p);
q=p(1)*poly(r);