Every 14 days, another language dies. There are many reasons for this. Some people think more common languages have more economic(经济的) if power. Because of this, young people choose to learn a common language as they think it is more useful. Another reason is that some languages aren't written down. Guujaaw is a leader of the Haida Nation, whose people have lived on the Queen Charlotte Islands, Canada, for more than 10,000 years. Their language is endangered. Traditionally, it wasn't written down, and, as a result, some people are worried that it will die one day. Guujaaw replies to this, "We talk to each other, listen, visit and believe in the spoken word. Expressing yourself without writing is natural. However, if Guujaaw's language and others like it are going to stay alive, writing may have to become part of their lives.

    When a language dies, a lot of knowledge dies with it. To begin with, language is a huge part of the culture of the people who speak it. Language allows speakers to say certain things: words that describe a cultural idea may not translate exactly into another language. Furthermore, many endangered languages have rich spoken cultures. Stories, songs and histories are passed on from older people to younger generations(一代). Elizabeth Lindsey, an expert in languages, emphasizes(强调) this. "When an elder dies, a library is burned," she says.

    Also, language death affects our knowledge about nature. Local people often have a deep understanding of plants, animals, and the relationship between them. David Harrison, a researcher on endangered languages, reflects, "80% of plant and animal species(物种)have been undiscovered by science. But that doesn't mean they're unknown to humans."

    Many languages are endangered. However, it's not too late. Children often grow up speaking two languages．“ No one… becomes richer by giving up… one language to learn another，” Harrison said. If children feel both languages are important, they will use both. So, it is necessary that children realize how useful their local language is.

如图所示，灯L标有“2.5V  1.25W”字样，R₁=15Ω，滑动变阻器R₂最大阻值为10Ω，当开关S₁闭合，S₂、S₃断开，滑片P滑至左端时，灯L恰好正常发光．试求：