The more common and generally accepted phrasing is weak in maths.
Explanation:
While both "weak at maths" and "weak in maths" can be understood, "weak in maths" is the standard and more grammatically correct way to express a lack of proficiency in mathematics. The preposition "in" is typically used to denote an area of study or expertise where someone lacks skill.
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"Weak in" indicates a deficiency within a specific subject or domain. This is the conventional usage for academic subjects.
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"Weak at" is sometimes used, but it can sound less formal or even slightly mocking, according to some style guides. Although this is subjective and dependent on context.
Therefore, for formal or neutral communication, "weak in maths" is the recommended option.
