题目

Crime has its own cycles, a magazine reported some years ago. Police records that were studied for five years from over 2,400 cities and towns show a surprising link between changes in the season and crime patterns. The pattern of crime has varied little over a long period of years. Murder reaches its high during July and August, as do rape(强奸) and other violent attacks. Murder, moreover, is more than seasonal: it is a weekend crime. It is also a nighttime crime: 62 percent of murders are committed between 6 pm. and 6 am. Unlike the summer high in crimes of bodily harm, burglary ha如图所示，正方体的棱长为1，，M是线段上的动点，过点M作平面的垂线交平面于点N，则点N到点A距离的最小值为__________。