Math functions in flash

A have a little problem using math functions with AS.
Some of them work, some don’t, and i can’t understand why.

Here’s an example:

y=x+2 – this._y = this._x +2 – this one works
y=x^2+2x+15 – this._y= this._x^2 + this_x*2 + 15 – dosen’t works

It is strange, does anyone have an idea how to use this kind of functions with the help of AS?