Graphing poly2sym([1 2 3])

I've used this to try to graph, but it gives me an inaccurate graph. Attached is a picture of the graph that I'm getting, it shows (0,0) which must not be true for this function.

How could i graph this polynomial?

function test()

     x=poly2sym([98 -280 235 -78 9]);
     ezplot(x)

end

Answer 1

[Star Strider]

What’s ‘inaccurate’ about it? What polynomial are you presenting to it, and what do you expect? The polynomial it evaluates is:

f(x) = 98*x^4 - 280*x^3 + 235*x^2 - 78*x + 9

[John Harris]

Look at it. There's clearly no point 0,0.

[Star Strider]

Of course there isn’t! It has an offset of +9. That’s the polynomial you gave it, at least as it (and I) interpret your vector.

What polynomial did you intend to give it?

[John Harris]

It's the right polynomial I think the zoom is just bad. I'm trying to zoom in on the roots. Do you know how to adjust the zoom. I tried ezplot(x, [-2*pi,2*pi,-2*pi,2*pi]) but its giving an error

[Andrew Newell]

You can play around with the axes using the axis command. For example,

axis([0 2 -1 1])

clearly shows the roots, which you can get using

solve(x)

ans =

           3/7
           3/7
 2^(1/2)/2 + 1
 1 - 2^(1/2)/2

Note the double root at 3/7, which is why it just touches the axis.

[John Harris]

Thank you this was just what I needed. I'm glad you told me there was a double root. I didn't notice but I needed to know