——This history book is of great value.——Exactly. ____________ can be enjoyed from it ____________ you have a deep understanding of it, however.
某班有学生52人，期末数学考试平均成绩是72分，有两名同学下学期要转学，已知他俩的成绩分别为70分和80分，求他俩转学后该班的数学平均分．
直流电动机的缺点之一是 ____．
