anyone have a clue why roots([1,6,9]) outputs complex numbers?

a = roots([1,6,9])

I was using roots to output the roots of the equation x^2+6x+9=0

this gives:

a =

-3.0000 + 0.0000i

-3.0000 - 0.0000i

when I use other values that create a quadratic equation with equal roots it seems to work fine, can someone explain to me why this is the case?

Answer 1

[Walter Roberson]

If you switch to format long you will see non zero imaginary parts. Those are caused by roundoff problems with solving the Vandermode system to get the roots.

Answer 2

[Fangjun Jiang]

a is correct. use real(a) to get what you want.

[Fangjun Jiang]

Okay, I get it. Good question!

>> a=roots([1 4 4])
a =
    -2
    -2