Geography on the flag of angola, the symbol with a half circle shape is part of three symbols, collectively chosen to relate to the flag of what former county?

The three symbols are: Cog wheel, machete and a star. Those represent the regional variation of worker's tools, and workers tools in similar combination were also seen on the flag of the Soviet Union - this is the correct answer. It's also a symbol of communist, which is the reason why both countries adopted it.