To say which is true, one would have to know whether you are studying mathematics at university; if you are, then both are true. (If you missed the point, 'true' is not the right word.) Try 'correct' or 'right'.
If someone asks "What's your major?", they already know that you study/are studying at university. I'd say that the best response is "Mathematics".
As to which of your sentences is better, if someone asks, "What do you do?", 1 is better.
If someone asks, "What are you doing these days?", 2 is better.
They both mean essentially the same thing.
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