Can I use theta for an unknown angle on a circle?
Yes, the Greek letter theta (θ) is one of the commonest symbols used to specify an angle. Just make it clear to which angle you are referring. A diagram is a good idea for this purpose or a few words such as ‘let θ be the angle between …’ If there are other angles to which you need to refer you can use other symbols such alpha (α), beta (β) etc. Sometimes θ’ and θ’’ are used. In theory you can use whatever you want as long as you make it absolutely clear what the symbol means. In practice there are conventions such as described and you wouldn’t use symbols such as pi (π) or infinity (∞) which have very specific meanings.
