A+B+C+D+E+F+G+H=A+B+C+D+E+F+ Angle HAF+ Angle GFA= the sum of the internal angles of a hexagon (I can't sign).
The idea is that because you are learning the internal angles and formulas of convex polygons, you must transform concave polygons into convex polygons. After connecting AF, we can find that angles can be converted, so we can convert complex angles into simple internal angles and formulas.