vain: implies either absolute or relative absence of value or worth: it's ~ to think we can alter destiny.


nugatory: suggests triviality or insignificance: a monarch with ~ powers.


otiose: suggests that something serves no purpose and is either an encumbrance or a superfluity: not a single scene in the film is ~.


idle: suggests a lack of capacity for worthwhile use or effect: it is ~ to speculate on what might have been.


empty, hollow: suggests a deceiving lack of substance, soundness, genuineness, or value: an ~ attempt at reconciliation (empty); a ~ victory that benefited no one (hollow).


vain无用: 泛指没有价值意义，nugatory不重要: 琐屑、不重要，otiose多余: 无用碍事或多余，idle无用: 泛指缺少效果作用，empty, hollow空洞: 虚假的缺少实质价值的空洞


记忆方法: 1) 首字母VNOIEH重组为NO HIVE没有蜂箱<==无用    hive蜂房蜂箱a structure made for bees to live in，还表示繁忙的地方
     
         2) 无用的意思是无价值或意义mean being without worth or significance.首字母VNOIEH重组为NO HIVE没有蜂箱<==无用