spizdi;l1

  For i := 1 to n-1 do
           begin
                if (xt < xi[i+1]) and (xt > xi[i]) then
                  begin
                    a :=((y[i+2]-y[i])*(x[i+1]- x[i])-(y[i+1]-y[i])*(x[i+2]-x[i]))/
    ((x[i+2]*x[i+2]-x[i]*x[i])*(x[i+1]-x[i])-(x[i+1]*x[i+1]-x[i]*x[i])*(x[i+2]-x[i]));
                    b :=(y[i+1] - y[i] - a*(x[i+1]*x[i+1] - x[i]*x[i])) / (x[i+1]-x[i]);
                    c :=y[i] - (a *x[i]*x[i] + b* x[i]);
                  end;
           end;
           kvadr := a*xt*xt+b*xt+c;
     end;