白费力气的用法和样例：
例句
-
大夏天推销大衣真是白费力气。
Trying to sell overcoats in midsummer is a real mug's game. -
他白费力气想抓住停在花上的蝴蝶。
He tried in vain to catch the butterfly on the flower. -
过去的事哭也是白费力气。
It is no use crying about the past. -
在无人岛上夸耀富有也是白费力气。
It is of no use boasting of being rich on a desert island.
