已知等差数列{a_n}的前n项和为S_n，且S₁₀=55，S₂₀=210．求数列{a_n}的通项公式；

There _____________ many people in the park.

阅读短文,判断正误。

    All the students in China are learning English, but how can they learn English well? Some students can know a lot about English, they can do some exercises, but perhaps they can't speak English loudly.

    If you want to know how to 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 by using (使用) it. You must listen to your teacher in class. You must speak English to your classmates everyday and also you must watch some English cartoons, sing some English songs, write some English diaries.... Then one day you may find you can speak very good English.