There are many benefits of singing in a choir. They range____the social: having the opportunity ____(make)new friends, to the physical benefits of learning to breathe ____(proper). Research has also shown that young people ____sing in choirs do better in their studies and have____tendency to be healthier.

    But the first question you should ask____(you)if you are considering joining a choir is “can you sing in tune?” It might seem obvious, but____you can, there is little point in seeking out a choir because you'll only get rejected. Every choir you apply to join is going to audition(面试)you. Don't worry if you can't read music; that can be taught later. If you ____(accept), you'll find choir practice to be hard work, but great fun, because they're social events as much as music training. You will learn to read music and how to sing with confidence. Your voice and technique will develop, probably quite quickly with proper ____(instruct), and you may just end up____(wonder)why you didn't do this years ago.

若x，y满足约束条件$\left\{\begin{array}{c}x+y\ge 1\\ -x+y\le 1\\ 2x-y\le 2\end{array}\right.$．

（1）求目标函数z=$\frac{1}{2}$x﹣y+$\frac{1}{2}$的最值；

（2）若目标函数z=ax+2y仅在点（1，0）处取得最小值，求a的取值范围．

（3）求点P（x，y）到直线y=﹣x﹣2的距离的最大值；

（4）z=x²+y²﹣10y+25的最小值；

（5）z=$\frac{2y-1}{x+1}$的范围．

    Thomas Edison lit up the world with his invention of the electric light. Without him, the world might still be in the dark. However, the electric light was not his only invention. He also invented the motion picture camera and 1200 other things. About every two weeks he created something new.
    Thomas Edison was born in 1847. He attended school for only three months. His mother taught him at home, but Thomas was mostly self-educated. He started experimenting at a young age.
    When he was 12 years old, he got his first job. He became a newsboy on a train. He did experiments on the train in his spare time. Unluckily, his first work experience did not end well. They fired him when he accidentally set fire to the floor of the train. Then Edison worked for five years as a telegraph operator, but he continued to spend much of his time in experimenting his first patent(专利权) in 1868 for a vote recorder run by electricity.
    Thomas Edison was totally deaf in one ear and hard of hearing in the other, but he thought of his deafness as a blessing in many ways. It kept conversations short, so that he could have more time for work. He always worked 16 out of every 24hours. Sometimes his wife had to remind him to sleep and eat.
    Thomas Edison died at the age of 84. He left a great many inventions that greatly improved the quality of life all over the world.

“背日喷乎水，成虹霓之状”描写的是“人工虹”、该现象产生的原因是光的（　　）