reciprocal      
 adjective  formal uk/rɪˈsɪp.rə.kəl/ us/rɪˈsɪp.rə.kəl/
 相互的;互惠的;报答的
A reciprocal action or arrangement involves two people or groups of people who behave in the same way or agree to help each other and give each other advantages.
 reciprocal      
noun [ C ] uk/rɪˈsɪp.rə.kəl/ us/rɪˈsɪp.rə.kəl/
 (数学)倒数
a number that, when multiplied by another number, results in 1