两个数相加,交换加数的____,和不变,这叫做____,用字母表示为____。
两个数相乘,交换因数的____,积不变,这叫做____,用字母表示为____。
Stop cutting trees, _______ the earth will become worse and worse.
Nowadays,“How do web celebrities ____ ____?” and “How to become a web celebrity” have become hot topics.
如图所示,用手拨动塑料尺,塑料尺____发出声音,用的力越大,塑料尺的____越大,改变塑料尺伸出桌面的长度,会使声音的____发生改变.