Hello Aspirant,

You can apply for the exam in subject mathematics, as a private candidate and complete the course. But, you may get a new marksheet which may only contain the result of maths in it or all the subjects. This depends on the board you give your exam with.  But, yes. It will be considered that you have studied maths and you can use or show both the marksheets  together when required.

Hope this helps.
Good luck!