挥笔

lǎo shī jiǎng guò lì tí zhī hòu huī bǐ zài hēi bǎn shàng xiě xià liǎng gè suàn shì ràng tóng xué men xiě zài cǎo gǎo běn shàng

老师讲过例题之后，挥笔在黑板上写下两个算式，让同学们写在草稿本上。

After the teacher has talked about the examples, write down two equations on the blackboard, and let the students write in the rough book.