Trill question

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I'm learning Beethoven Sonata in G op49 no2.  In measure 109 (as well as other places but i am learning the piece out of order) there is a trill in the right hand, the note A with the letters Tr above it.  So far i have been learning pieces that give you notation at the bottom of the page on how to play trills, what notes and rhythm to use.  When i see the Tr is there a standard way to play it?  Right now i've been playing it B-A-G-A and the second A lands on the 3rd beat in the measure.  Like i said, i don't know if this is the correct way to do this or if there is a standard way.

[b0mbtrack]

i just read that you should start on the note above the main note (A) and alternate between the two. B-A-B-A-B-A.  I guess i'll just do it that way in 16th notes.