How to find the solution of inverse function ?

Hello, I have some question for solving matlab.

if x = [1 2 3 4 5 6 7] and y = [3.6 1.8 1.2 0.9 0.72 1.5 0.51429]

I want to find f(x) = 1.7 and inverse of f(1.7) (quadratic form)

The code I implemented is

>> x = [1 2 3 4 5 6 7]; y = [3.6 1.8 1.2 0.9 0.72 1.5 0.51429];

>> p = polyfit(x,y,2);

>> syms x;

>> fx = p(1)*x^2 + p(2)*x + p(3);

>> gx = finverse(fx,x);

>> gx(1.7) // error! (The index exceeds the number of array elements (1).)

Why does gx(1.7) throw an error? What should I do for getting gx(1.7) ?

Answer 1

[Stephan]

x = [1 2 3 4 5 6 7]; 
y = [3.6 1.8 1.2 0.9 0.72 1.5 0.51429];
p = polyfit(x,y,2)
syms x
fx(x) = poly2sym(p)
gx(x) = finverse(fx)
sol = gx(1.7)
sol_num = double(sol)

[SeokWon CHOI]

Thank you, sir! solve it !

but I don't know why my code has error.

and what is the poly2sym? I don't learn about this function.

Could you explain me?

[Stephan]

poly2sym creates symbolic polynomial from vector of coefficients - this is what you need to do here. Also there are 2 different symbolic data types namely sym and symfun, which lets you calculate values with symbolic functions.

https://de.mathworks.com/help/symbolic/create-symbolic-functions.html