Dolores Huerta has worked hard most of her life to help other people. She has helped change things so that farm workers can have a better life.
      Dolores grew up in California. She was a good student and liked school. After she finished high school, she went to college and studied to be a teacher. After she became a teacher, Dolores noticed that many of her students were not getting enough food to eat. Some of them wore very old clothes. Dolores wondered how she could help them.
        Dolores decided to stop teaching so that she could spend more time helping the farm workers and their families. One thing she wanted to do was to get more pay for farm workers so they could buy their children the things they needed.
Dolores knew that many farm workers moved often from one place to another to help pick different kinds of fruits and vegetables. She began talking and writing about these workers. Even people who lived far from California read what Dolores wrote.
      Getting higher pay for the farm workers was not easy. Dolores worked hard to make sure that farm workers got good pay for their work. She knew that nothing would change unless people made new laws to help the workers. Through all her hard work, new laws were made that gave farm workers good pay.
     Dolores Huerta has worked for more than 30 years in many different ways to make life better for working people. She has shown how much one person can change things.

体育课上，老师要测量一下每位同学从一楼登上三楼的功率．为此，需要测量一些物理量，如：①三楼地面到一楼地面的高度；②从一楼到达三楼所用的时间；③每个同学的质量或体重； ④一楼到三楼楼梯的长度．上述物理量中必须测量的是____ ．

已知方程 的两个根为 、 ，那么 的值（    ）