Animals are our friends. We can find different animals here and there. Some animals live in big forests(森林) and some animals live with man.

    Animals can help us a lot. Elephants can do some heavy work for us. And dogs can look after the home. The kids like dogs very much. Cats are women's favorite animals.

We can eat their meat. We can make coats and shoes with their skins(皮). We can get milk from them. So man can’t live without animals.           

若xⁿ=4，yⁿ=9，则（xy）ⁿ=____ 

土星外层上有一个环（如图），为了判断它是土星的一部分还是土星的卫星群，可以测量环中各层的线速度v与该层到土星中心的距离R之间的关系来判断（   ）