在等比数列{a_n}中，a₂，a₆是方程x²﹣34x+64=0的两根，则a₄等于（   ）

近年来，雾霾天气逐渐增多。对此，有网友调侃：“遛狗不见狗，狗绳提在手；见绳不见狗，狗叫我才走。……世界上最远的距离，是牵着你的手，却看不见你的脸。”这告诉我们（　　）

用方框中所给单词的适当形式填空，使短文通顺、正确、连贯（每个单词限用一次）。                                  

get,  look,  some,  year,  a,  write,  by,  tomorrow,  work,  other

      Tommy is a schoolboy. He's thirteen ____  old. He's in Class 3, Grade 1. He always____  up early and he often goes to school ____ bike. He likes English, Chinese and some ____ subjects. He enjoys singing and dancing. But now he is having ____ English class.____ ! some of the students are reading books and some of the students are____  the new words. They are ____  hard . But ____  is Saturday. They don't have ____ classes. He and his friends will go to the movies. I think they'll have fun.

计算：2×（﹣$\frac{1}{2}$）=____．

将一处原始森林开辟为一个国家森林公园，为了继续维持森林的生态平衡，应当采用的措施是（　　）