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 However, nonspecialists (including many doctors) still tend to use an older perspective, now seen as outdated by experts.
Consider the latest government statistics: 1  Nearly 23 million Americansâalmost one in 10âare addicted to alcohol or other drugs. 2  More than two-thirds of people with addiction abuse alcohol. 3  The top three drugs causing addiction are marijuana, opioid (narcotic) pain relievers, and cocaine.