notice the picture below
thus, using the "far arc - near arc" theorem [tex]\bf \measuredangle A=\cfrac{\widehat{BDC}-\widehat{BC}}{2}[/tex]
now, we know what arcBC is, what's arcBDC? well, keep in mind, a circle only has 360°, so if arcBDC is just 360 - arcBC
