Of course, not necessarily ... Lagrange multiplier method is actually calculated according to the necessary conditions, which is of course insufficient. But many problems often only calculate a point with a derivative of 0. According to practical significance, if the problem has a maximum (non-boundary point) at this time, then the point with derivative of 0 is the maximum point. As a college entrance examination, you naturally don't need to pay attention to too many complicated situations, as long as you can be constrained by equations.