脱式计算．

$\frac{6}{7}-\frac{2}{9}÷\left(\frac{1}{2}+\frac{2}{3}\right)$

苏格拉底说：“我是一只牛虻……这个国家好比一匹硕大的骏马，可是由于太大，需要一只牛虻叮叮它，使它的精神焕发起来。”这表达了怎样的人生价值观（    ）

（2016•山东）已知数列{a_n}的前n项和S_n=3n²+8n，{b_n}是等差数列，且a_n=b_n+b_n+1．

    Tea is an important part of Chinese tradition. Tea has very close relationship to Chinese culture.

    It is said that Chinese tea was discovered by King Shen Nong. People say that he lived 5，000 years ago. One summer day, while he was visiting a faraway part of his country, he felt very thirsty. The servants began to make water hot enough for him to drink. Dried leaves from a near plant fell into the water. The king drank some, and found it could make him feel less tired. Also as a scientist, Shen Nong was interested in the new drink. And so, tea was created in 2737 BC.

    Drinking tea has many advantages. It makes people feel less tired, clears heat inside the human body and helps people become slimmer. As you add a cup of tea to your daily life, please check the following helpful advice.

    Drink it hot. Tea oxidizes(氧化)quickly, so it is suggested that you drink it hot.

    Do not drink too much strong tea. It will probably be harmful to your stomach and make you feel sick if you make the tea too strong.

    The best time to drink tea is between meals. It may not make you have a strong wish to eat when your stomach is empty, or cause stomachache when your stomach is full.

    Do not drink with medicine. It may change the medical result. You can drink tea two hours after you take medicine.

    Green tea is the best choice for office workers. Why? Because green tea helps stop the bad result of the computer.