難題的英文：
a difficult problem; an arduous problem; a tough question

difficult是什么意思：
adj. 困难的；执拗的；不易相处的
  • The exercise is that difficult!
    练习题那么难！
  • You must admit the task to be difficult [ that the task is difficult ].
    你应当承认这项任务是艰巨的。
  • You must admit the task to be difficult [that the task is difficult].
    你必须承认这项任务是困难的。
problem是什么意思：
n. 难题，困难；逻辑题
  • It was a complex problem.
    这是一个复杂的问题。
  • Is the problem that easy?
    问题有那么简单吗?
  • This is a prime problem.
    这是一个有关质数的问题。
arduous是什么意思：
adj. 费力的，辛勤的，险峻的
  • Success is the fruit of arduous striving.
    成功是奋斗的结果。
  • They are very glad that they have been assigned the most arduous task .
    他们很高兴被派给最艰巨的任务。
  • His timidity disinclined him from such an arduous enterprise
    他的怯懦使他不愿从事一项如此艰巨的事业。

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