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 However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
Even though support for legalization has doubled in the past 20 years or so, usage has remained relatively flat [source: Saad ]. According to a July 2013 Gallup poll, roughly 38 percent of Americans had tried marijuana, and 7 percent, or 22 million people, partook of the drug.