Hello,
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Yes, of course maths is useful.Not only in UPSC examination, even in group exams in most of the government/service examinations maths is useful.
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You will not be getting graduation level mathematics, only till intermediate level mathematics will be appearing in the examination.
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If you are a maths student, then just know the important topics and practice, no need of Institute.
But if you are weak in mathematics, you should go for coaching, if not you can't solve the problems