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Hi I have a question on 9.21 Geometry II-A

 
 
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Hi I have a question on 9.21 Geometry II-A
by Bober Yang - Thursday, 8 July 2021, 9:27 AM
 

yeah so for this one I still don't get the explanation. I don't know where is the 30-60-90 triangle or the big picture in general. I am super confused. can I get some help?