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 However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
Marijuana is the most heavily used drug in the country â by their 20s, 56% of Americans have tried it â but only 16% of people who are in addiction treatment report that marijuana is their primary drug. In contrast, just 2% of young adults have ever tried heroin, but heroin addicts make up 14% of treatment admissions.