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 However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
(See Table 6.) For example, California contains 13.25% of annual marijuana users in the United States according to the National Survey on Drug Use and Health but also accounts for 38.68% of the marijuana produced in the United States, a ratio of production to use of 2.92.