如图，∠AOB=60°，PD⊥OA于D，PE⊥OB于E，且PD=PE，则∠1=____.

（1）写出一个化学方程式，其转化属于“氮的固定”：　 ____　．

（2）写出实验室制取氨气的化学方程式：　 ____　，实验室可以用 ____．

（3）NO₂与水反应的化学方程式为　 ____　，该反应中氧化剂与还原剂的物质的量之比为　 ____　．

（4）氢氟酸应存放在 ____中，因为　 ____　（请用化学方程式表示原因）

    Many Chinese believe____(eat) pig brains will increase their intelligence and some Americans regularly eat____ (cook) eggs to keep their minds sharp.  ____ doctors don't recommend eating____ raw eggs or animal brains. Then, what food is good for your brain? Scientists have found a relationship  ____ diet and a healthy brain. Generally speaking, fish is ____brain food. Besides fish, you should introduce lots of dark green leafy vegetables and colorful fruits into your diet,____ most people lack the special vitamins____ these brain food provide. If you eat brain foods____(regular), they can increase brain power — help you pay attention, keep you motivated, improve your memory and ____(less) stress from studies. Then you can do much better in your test.

以长为3cm，5cm，7cm，10cm的四条线段中的三条线段为边可以画出三角形的个数为(    )