How do I get the x vector from a fifth degree function with a given y vector ?

Hello !

I have a fifth degree function as shown below:

F = @(x) -0.1304*x.^5 +0.2909*x.^4 - 0.2454*x.^3 + 0.042*x.^2 + 0.0693*x + 1.3332

and i have a vector for the given y values:

y = [1.3579 1.3571 1.3571 1.3567 1.3568 1.3565 1.357 1.3571 1.3571 1.353 1.3405 1.3361 1.3341]'

I want to know how to get the x solution for this problem.

Thanks in advance.

Answer 1

[KSSV]

P = [-0.1304 0.2909 -0.2454 0.042 0.0693 1.3332] ;

y = [1.3579 1.3571 1.3571 1.3567 1.3568 1.3565 1.357 1.3571 1.3571 1.353 1.3405 1.3361 1.3341] ;

iwant = zeros(length(y),5) ;
for i = 1:length(y)
    P1 = [P(1:end-1) P(end)-y(i)] ;
    iwant(i,:) = roots(P1) ;
end

Answer 2

[Stephan]

Hi,

if you are interested in only the real solutions you can use:

y = [1.3579 1.3571 1.3571 1.3567 1.3568 1.3565 1.357 1.3571 1.3571 1.353 1.3405 1.3361 1.3341]';
F = @(x) -0.1304.*x.^5 +0.2909.*x.^4 - 0.2454.*x.^3 + 0.042.*x.^2 + 0.0693.*x + 1.3332 - y';
result = fsolve(F,ones(1,size(y,1)))

Result is:

result =

  Columns 1 through 7

    1.0221    1.0314    1.0314    1.0359    1.0348    1.0380    1.0325

  Columns 8 through 13

    1.0314    1.0314    1.0720    1.1567    1.1792    1.1885

Best regards

Stephan