题目

  You get anxious if there’s no wi-fi in the hotel or mobile phone signal up the mountain. You feel upset if your phone is getting low on power, and you secretly worry things will go wrong at work if you’re not there. All these can be called “always on” stress caused by smart phone addiction.     For some people, smart phones have liberated them from the nine-to-five work. Flexible working has given them more autonomy(自主权) in their working lives and enabled them to spend more time with their friends and families. For many others though, smart phones have become tyrants13．已知直线l：y=ax-a-2，其中a为实数（常数），可知直线l必经过定点（1，-2）；过点P（-1，0）作直线l的垂线，垂足为M，点O为坐标原点，则线段OM长度的最小值为$\sqrt{2}$-1，最大值为$\sqrt{2}$+1．