阅读理解

   Look! This is my new schoolbag. It's big. It's blue and white. I have an English book, a Chinese book, a math book, three notebooks and a pencil-case in it. In my pencil-box, you can see a ruler, two pens, seven pencils and a notebook. Oh, where is my schoolbag? It's on my chair.

    We are all learning English, but how can we learn English well? A student can know a lot about English, but maybe(可能) he can't speak English. If you want to know how to (如何) swim, you must get into the river. And if you want to be a football player, you must play football.
So, you see. You can learn English only by(通过) using(使用)it. You must listen to your teacher in class. You must read your lessons every day. You must speak English to your classmates and also you must write something sometimes. Then one day, you may find your English very good.

已知△ABC的三边分别为a，b，c，a=n²﹣16，b=8n，c=n²+16（n＞4）．

求证：∠C=90°．