When something great happens to a friend, a family member or a classmate, congratulations are in order! However, some people don't know how to congratulate others. Here are some ways to show your congratulations.

Say “congratulations” with a smile. Remember to express your congratulations with a winning smile. The expression on your face is pretty important. If you say “congratulations” with a serious look on your face, the person will probably think you aren't actually all that happy for him or her.

    Send an en email or a card. Besides saying “congratulations in person, you might want to send a congratulatory note. You can send a simple email or write a message on a card. You can buy some beautiful cards in the store.

    Send a gift. If the occasion is special enough, you can send a gift. You don't need to buy something expensive. Try to buy a suitable gift. All in all, choose a gift that lets the person know you want to celebrate with him or her.

    Spread the word. You can help spread the person's joy by telling other people the good news, so they can congratulate the person as well. If the person prefers to keep the good news private, don't spread the word.

第1个等式：x₁= $\frac{1}{1\times 3}=\frac{1}{2}\left(1-\frac{1}{3}\right)$ ；第2个等式：x₂= $\frac{1}{5\times 3}=\frac{1}{2}\left(\frac{1}{3}-\frac{1}{5}\right)$ ；

第3个等式：x₃= $\frac{1}{5\times 7}=\frac{1}{2}\left(\frac{1}{5}-\frac{1}{7}\right)$ ；第4个等式：x₄= $\frac{1}{9\times 7}=\frac{1}{2}\left(\frac{1}{7}-\frac{1}{9}\right)$ ；

则x_l+x₂+x₃+…+x₁₀=____．

如图所示，在物理实验中小明把鸡蛋放入盛水的杯中，鸡蛋沉在杯底如图甲杯中加盐，鸡蛋悬浮如图乙，再加盐，鸡蛋漂浮如图丙，在这三种情况中，各物理量之间的关系正确的是（　　）