Sometimes talent runs in families. Let's look at some famous (著名的) brothers and sisters.

    Andy Murray and Jamie Murray are very famous brothers. They are both professional (职业的) tennis players and they play in competitions all over the world. Jamie is older than Andy, but only by about a year. They look very similar. They are the same height and they have the same dark hair. Jamie became a professional player in 2004, one year before his younger brother Andy. But Andy wins more competitions than his brother!

    Kylie Minogue and Dannii Minogue are famous Australian sisters. Kylie began her career (事业) as an actress on Australian TV, then she began singing when she was 19 years old. She soon became famous all over the world. Dannii is younger than Kylie. She is also taller than her sister and she has darker hair. She began singing after Kylie, but she is also very famous. Dannii is also famous as a judge (评判员) on TV talent shows. Sometimes Kylie and Dannii sing together, and they sing really well.

某同学利用托盘天平、玻璃杯、水、细棉线、记号笔，测出一块小石块的密度，他主要实验步骤如下：

（1）用调好的天平测出石块的质量m₁，并记录在表格内；

（2）在玻璃杯中倒入适量水，用细棉线系住石块缓慢地 ____在水中，标记此时水面的位置，用天平测出玻璃杯、水和石块的总质量为m₂，并记录在表格内；

（3）取出石块，再向玻璃杯中加水，使水面回到标记的位置，然后再用天平测出玻璃杯和水的质量m₃，并记录在表格内；

（4）石块的密度的表达式为ρ_石= ____