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 However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
And, the marijuana they smoke is much more potent than it was in the 1970s, with far higher levels of THC, the main mind-altering ingredient. The higher the THC levels, the more brain changes there are and the more there is the risk for addiction, the University of Wisconsin's Lisdahl says. She says more teens and young adults are smoking marijuana in states that have made the drug available for medical use.