This is the problem which comes from trying to be too smart. You should not use Greek prefices if you do not understand them. There is no such thing as a tridecagon: a 13-gon is a triskaidecagon whereas a 30-gon is a triacontagon. Stick to simple forms through which you can communicate rather than try and be pretentious!
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Because there are 13 angles, using (n-2)*180º, you can plug in n as 13 and find there are 1980º in a tridecagon.