垂直向力牙合力最易引起牙周组织损伤。
参考答案:错
In today’s fast changing and competitive world, to be able to learn faster and more effectively is not merely desirable; it is ________.
A.essential
B.worthwhile
C.challenging
D.sufficient
计算:(1)6a5b6c4÷(-3a2b3c)÷(2a3b3c3).
(2)(x-4y)(2x+3y)-(x+2y)(x-y).