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function [P]=quadraticapprox(x,f)
n=length(x);m=length(f);
if m<>n then
error('linreg - Vectors x and f are not of the same length.');
abort;
end;
s1=0;
s2=0;
for i=1:n
s1=s1+x(i)*f(i);
s2=s2+x(i)^2*f(i);
end
c0=det([sum(f) sum(x) sum(x^2);s1 sum(x^2) sum(x^3); s2 sum(x^3) sum(x^4)])/det([n sum(x) sum(x^2);sum(x) sum(x^2) sum(x^3); sum(x^2) sum(x^3) sum(x^4)]);
c1=det([n sum(f) sum(x^2);sum(x) s1 sum(x^3); sum(x^2) s2 sum(x^4)])/det([n sum(x) sum(x^2);sum(x) sum(x^2) sum(x^3); sum(x^2) sum(x^3) sum(x^4)]);
c2=det([n sum(x) sum(f);sum(x) sum(x^2) s1; sum(x^2) sum(x^3) s2])/det([n sum(x) sum(x^2);sum(x) sum(x^2) sum(x^3); sum(x^2) sum(x^3) sum(x^4)]);
X=poly(0,"X");
P=c2*X^2+c1*X+c0;
endfunction
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