Hi everyone, I want to find the polynomials from root. I want to get a value without decimal, how is that possible? Please find the example below for more clarification.

Question

Supermankid 2017-9-9

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/355838-hi-everyone-i-want-to-find-the-polynomials-from-root-i-want-to-get-a-value-without-decimal-how-i

评论： Supermankid 2017-9-11

Hi everyone,

I want to find the polynomials from root. I want to get a value without decimal, how is that possible? Please find the example below for more clarification.

>> poly([ -0.1667 + 1.2802i -0.1667 - 1.2802i])

ans =

1.0000 0.3334 1.6667

Which could be x^2+0.3334x+1.66667=0

But how do I obtain values without decimal? 3x^2+x+5=0 is same as x^2+0.3334x+1.66667=0

my question is how do i convert

ans =

1.0000 0.3334 1.6667

TO ===>

ans =

3 1 5

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Stephen23 2017-9-9

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/355838-hi-everyone-i-want-to-find-the-polynomials-from-root-i-want-to-get-a-value-without-decimal-how-i#answer_280881

编辑：Stephen23 2017-9-9

在 MATLAB Online 中打开

>> V = poly([ -0.1667 + 1.2802i -0.1667 - 1.2802i])
V =
            1       0.3334       1.6667
>> [N,D] = rat(V,0.001) % get numerator and denominator
N =
     1     1     5
D =
     1     3     3
>> N = prod(D).*N./D % convert all to same denominator
N =
     9     3    15
>> d=N(1); for k=N(2:end), d=gcd(d,k); end % find GCD
>> N = N./d % divide by GCD
N =
     3     1     5

3 个评论
显示 1更早的评论隐藏 1更早的评论

Stephen23 2017-9-11

编辑：Stephen23 2017-9-11

在 MATLAB Online 中打开

@Supermankid: okay, lets have a look at this line in detail:

N = prod(D).*N./D % convert all to same denominator

As you can see from my comment, the goal of that line is to convert all of the numerators to have the same denominator. The simplest denominator to calculate (although likely not the smallest) is to make the new denominator the product of all of the denominators, thus prod(D). But observe that the three rational fractions returned by rat already have different denominators: how can we account for this? Simply by dividing the new common denominator by the existing denominators, thus

>> prod(D)./D
ans =
   9   3   3

is the scaling required to ensure that all numerators have the same denominator. So multiply this by N and you have all of the numerators given with the same denominator:

>> N.*prod(D)./D % and of course this
ans =
    9    3   15
>> prod(D).*N./D % is equivalent to what I used
ans =
    9    3   15

And you can check it too:

>> N = N.*prod(D)./D; % convert all to same denominator
>> [N2,D2] = rat(N./prod(D))
N2 =
   1   1   5
D2 =
   1   3   3

which is the same as the original N and D returned by rat. So we have simply adjusted the numerator values to give exactly the same fractions, but using just one common denominator

PS. remember to accept the answer that best resolves your original question. This is the easiest way for you to thank the volunteers to help you.

Supermankid 2017-9-11