题目

在短文的空格内填入适当的词,使其内容通顺，每空格限填一词，词首字母已给。     Big schoolbags have been a serious problem for students for a long time. Maybe your schoolbag is too h   1   to carry, and it troubles you a lot when you want to find a book out to read. Now an e-textbook will help you.     It is said that e-textbooks are going to be used in Chinese middle schools. An e-textbook, in fact, is a small c   2   for students. It is much 1 3   than a usual schoolbag and easy to carry. Though it is as small as a book, it can h   4   all t已知平面上三个向量a、b、c的模均为1，它们相互之间的夹角均为120°.（1）求证：（a-b）⊥c；（2）若|ka+b+c|＞1(k∈R),求k的取值范围.