如图所示，手用F₁的力直接将物体B匀速提升h，F₁做功为300J；若借助滑轮组把B匀速提升相同高度，滑轮组机械效率是30%，则F₂做功为____J．

若一个正多边形的一个外角是40°，则这个正多边形的边数是（     ）

老年痴呆症患者的脑血管中有一种特殊的β一淀粉样蛋白体，它的逐渐积累可能导致神经元损伤和免疫功能下降．某些基因的突变会导致β一淀粉样蛋白体的产生和积累。下列技术不能用于老年痴呆症治疗的是(    )

阅读短文，判断下列句子是否正确。
    Ann went to Qingdao with her friends on her holiday. They went there by train. They went to the beautiful beach. The weather was sunny and hot. So they had great fun playing in the water.They ate lots of fresh fish for lunch. In the afternoon, they rode bikes. Jack fell off his bike and hurt his leg. So Jack stayed at the hotel and watched TV. On the second day, Ann took many pictures of the sea. And she bought some gifts for her parents. Anyway, they had a good time.

﹣5的倒数是（   ）