Curve Fitter

The following function fits exactly, but has the wrong asymptotic behaviour $$y= 0.75x^4-2.5 x^3+0.75x^2+3x$$

The following cubic nearly fits $$2 y = x^2(2x-9)+11x$$

This following quintic exactly fits $$4y = 4x^5-21x^4+42x^3-45x^2+28x$$

The differential set to zero gives $$ 20x^4-84x^3+126x^2-90x+28 = (x-1)(x-2)(20x^2-24x+14)=0 $$

This is by choosing the coeff of $x^5$ to be $1$

This curve only has 2 real turning points.

You can see these graphs on the attached xls