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）

根据短文理解完成下列各题。

    Because earthquakes happen without warning, it's important to take steps now to prepare. Because you don't know where you will be when an earthquake happens, prepare some supplies for your home, workplace, and car.
1. Water.
    A person needs at least 1/2 gallon of water daily just for drinking. Store at least 1 gallon of water per person per day and be prepared for a 72-hour period. It is suggested that you buy bottled water. Keep bottled water in its original container and do not open it until you need to use it. Also, do check the “use by” date.
2. Food.
    It's always a practical idea to keep a supply of non-perishable(不易腐烂的) food on hand. Have an enough supply of canned food, powdered milk, dried fruits, non-salted nuts and canned juices for at least 72 hours.
3. Flashlights(手电筒) and spare batteries.
    Keep a flashlight beside your bed, at your place of work, and in your car. Do not use matches or candles after an earthquake until you are sure that no gas leaks.
    Prepare a battery-powered radio and spare batteries. Most telephones will be out of order, so radios will be your best source of information.
4. Clothes.
    If you live in a cold place, you must think about warmth. You might not have heat after an earthquake. Think about your clothing and bedding supplies.

丽丽赴台湾探亲前查阅到台湾的相关资料，你认为符合事实的是