There are no weird letters in maths.
Greek mathematicians were responsible for a significant amount of the early mathematical work and they, naturally used Greek letters. Many of these letters are still used today. Just because you are not familiar with characters in the Greek alphabet does not make them "weird letters".
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Math uses letters to indicate that specific numbers are not known or as an abbreviation for certain constant.
In ordinary everyday letters . . . pi In Greek letters . . . Π
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Letters used in math are called variables.
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