题目

Today we had a chemistry test. I found the test difficulty, but I tried hardly to do it. Suddenly Mary, my best friend, asking me to let her to copy my answers. After think for some time, I let her copy my answers. But after the test, all of us were called to the teacher's office. The teacher was angry because we had same answers in the tests. We were warned not to cheat again so she would need to see our parents. I was very upset. I didn't cheat. I was just helping a friend. Why does she punish me?已知x，y是有理数，且|x-6|与（x-2y）2互为相反数，求2xy+13的值．