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I think 'will be doubled' and 'will double' has the same meaning, like
The cost will be doubled
The cost will double
They are the same setnece, isn't it?
And one more thing, if there is specific time in future tense, should I
use only future perfect?
The cost will have been doubled by the end of this year.
or, just simple futre is ok?
The cost will be doubled by the end of this year.
Always appreciate your help.
They are more or less the same.
We use the future perfect for things that will be complete by a certain time in the future. You can say 'the cost will doublt by the end of the year'. ;-)
Actually, 'will be doubled' is in the simple future passive. Whereas, 'will double' is in the simple future active. Here, the distinction is made only in terms of the the agent/actor that is responsible for the action indicated by the verb 'double'. In 'will be doubled', there is an implicit indication of an external agent 'doubling' the cost. 'Will double' implies the 'doubling' of the cost as a result of a variation in the internal machanics of the entity on which the cost is based. The future perfect has the construction: subject + will + have + past participle.
Best wishes :)