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 However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
The survey found smaller but still statistically significant decreases in cocaine use by youths ages 12 to 17. Marijuana use was flat over the same decade: In 2015, 7 percent of 12- to 17-year-olds said they had smoked the drug, roughly the same number in 2005.