[ML] Choose of Polynomial regression function

Polynomial regression:
    cubic function instead of quadratic function.
   (since we don't want the value y goes down while x increases.)
       
replace:
    h(x) = ϴ0 + ϴ1X1 + ϴ2X2...
to:
    h(x) = ϴ0 + ϴ1X1 + ϴ2(X2)^2 + ϴ3(X3)^3
   
easy mapping way:
    X1 = X1
    X2 = X2^2
    X3 = X3^3
   
Beware the range of the feature.
e.g:
    X1 -> 10
    X1^2 -> 100
    X1^3 -> 1000
   
Or using a square root function as the regression function,
since sqaure root function increase slowly.
* Also, be sure to adjust the range by square root the original range.*