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 However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
Part II. Marijuana remains the third most popular recreational drug of choice in the United States despite 60 years of criminal prohibition. According to government figures, nearly 70 million Americans have smoked marijuana at some time in their lives. Of these, 18 million have smoked marijuana within the last year, and ten million are regular marijuana smokers.