①若 $\overrightarrow{a}\cdot \overrightarrow{b}\text{=}\overrightarrow{b}\cdot \overrightarrow{c}则\overrightarrow{a}\text{=}\overrightarrow{c}$ ．

② $\overrightarrow{a}$ =（1，1）， $\overrightarrow{b}$ =（2，x），若 $\overrightarrow{a}+\overrightarrow{b}$ 与 $\text{4}\overrightarrow{b}\text{-}2\overrightarrow{a}$ 平行，则x=2．

③非零向量 $\overrightarrow{a}$ 和 $\overrightarrow{b}$ 满足| $\overrightarrow{a}$ |=| $\overrightarrow{b}$ |=| $\overrightarrow{a}\text{-}\overrightarrow{b}$ |，则 $\overrightarrow{a}$ 与 $\overrightarrow{a}+\overrightarrow{b}$ 的夹角为60°．

④点A（1，3），B（4，﹣1），与向量 $\overrightarrow{AB}$ 同方向的单位向量为（ $\left(\frac{3}{5}\text{,-}\frac{4}{5}\right)$ ）．

其中真命题的序号为____．（写出所有真命题的序号）

① 人体   ② 组织  ③ 细胞  ④ 系统  ⑤ 器官

阅读下面材料，在空白处填入适当的内容(1个单词) 或括号内单词的正确形式。将答案填写在答题卷的相应位置。

Let go of the problems

    There are many teenagers who have too much pressure. Last Wednesday, Mr. Trent began his psychological health class ____ holding up a glass of water. He asked the students____would happen if he held it up for a day.____ student said the teacher's arm would ache or go numb. He might have to go to hospital! All the students laughed.

     "But during the time, did the ____(weigh) of the glass change?" asked Mr. Trent. "Then what caused the pain and numbness?" 

     The students ____(be) puzzled.

      "What should I do now ____(get) rid of the pain?" asked Mr. Trent again.

      "Put the glass down!" said another student.

      "____(exact)!" said Mr. Trent. "Life's problems are something ____ caused the pain like this glass. Hold them for a few ____(minute) in your hand and they seem OK. Think of them for a long time and they begin to ache. The longer you hold them, the more painful you feel."

     It's important to think of the challenges or problems in your life. But what's even ____(important) is to put them down at the end of every day.

  ____ + ____ = ____

当今世界，有220多个国家和地区，2500多个民族，6000多种语言。不同民族、不同历史文化背景的人们，共同创造了各具特色、丰富多彩的文化。不同文明之间持续开展对话、交流，最终融汇成人类文明奔流不息的长河。这表明（    ）

①人民群众是文化创造的源泉

②文化交流使得各民族文化趋向统一

③文化既是民族的又是世界的

④文化多样性是人类文明进步的动力