For a function of two variables z=f(x,y), the gradient is the two-dimensional vector <f_x(x,y),f_y(x,y)>. This definition generalizes in a natural way to functions of more than three variables. For the function z=f(x,y)=4x^2+y^2.
By this definition, about 10% of people who smoke marijuana become addicted to it. (More on Time.com: PHOTOS â Cannabis Conventions). However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
Not everyone will just use the drug once either; research is showing that people do become addicted to synthetic marijuana products. Because the substance is still relatively new, not much is known about how or why synthetic marijuana addiction occurs, but the anecdotal evidence of its effects is piling up.