无级变速是在变速范围内任意连续地变换速度，性能优于传统的档位变速器．如图所示是截锥式无级变速模型示意图，两个锥轮中间有一个滚轮，主动轮、滚轮、从动轮之间靠着彼此之间的静摩擦力带动．当位于主动轮与从动轮之间的滚轮从左向右移动时，从动轮转速降低，滚轮从右向左移动时从动轮转速增加．当滚轮位于主动轮直径D₁、从动轮直径D₂的位置上时，则主动轮转速n₁，从动轮转速n₂之间的关系是（   ）

已知△ABC中，AB=AC，则cosB+cosA的最大值为____ 

    All of us know that breakfast is very ____ 'nesəsərɪ to us. It is____of the most important meals of the day. To keep ____ (health), everyone should have a good breakfast.____ (eat)the right food is good for our health. We can have eggs, bread, milk and other food ____ breakfast.

    Many students go to school without breakfast.____  bɪ'kɒz they get up late or their parents don't cook for them. It's ____ for their health.

    Without breakfast children may not____grəʊfast and may not study and play very well at school. They may be easier to get____(ill). And when they're ill, it will____(take) them a long time to recover（恢复）. I think everyone should have some good eating habits.