Cambridge gives 'focus on' https://dictionary.cambridge.org/dictionary/english/focus-sth-on-sb-sth
One of the Oxford's textbook of English gives 'focus at' as in the pic.
On one of the English reference forum a native speaker said it's unusual to use 'focus on' to him.
Well where is the right approach to use it?
The verb focus takes the preposition on when it is needed. In your example at has nothing to do with focused. In your sentence at is a part of the phrase at work.
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The verb focus takes the preposition on when it is needed. In your example at has nothing to do with focused. In your sentence at is a part of the phrase at work.
CB