Tom's school basketball game is next month. His class 1 a new player. His teacher asks him to be one. of the players 2 he is tall (高的). But Tom doesn't like 3 sports. He can't play basketball 4 and he thinks it is 5 for him, so he calls his friend Edward to help him. Edward says he'll help Tom 6 his basketball skills (技巧). They have about 20 days to prepare for (为……做准备) the game, and in the next three 7 they play basketball for 2 hours a day. They don't have much 8 at school, so they play it after the classes 9 .Tom is tired (疲倦的) but happy every day. Now, Tom loves 10 He thinks it is relaxing and fun.

用6N的水平拉力，使小车在水平桌面上做匀速直线运动，现将拉力撤掉，小车在桌上做减速运动，此时小车受的摩擦力是    (    )

继“单独二胎”政策全面实施一年多后，2015年10月，中共十八届五中全会公报允许普遍二孩政策．该政策实施的主要目的是（　　）

①可以迅速提高人口素质

②可以解决人口增长过快的问题

③有利于环节劳动力短缺的矛盾

④有利于缓解人口老龄化带来的问题．

已知$x\in \left(2k\pi -\frac{3}{4}\pi ,2k\pi +\frac{\pi }{4}\right)\left(k\in Z\right)$，且$\cos \left(\frac{\pi }{4}-x\right)=-\frac{3}{5}$，则cos2x的值是（  ）