Tests whether a point lies in a region. Disk and Ball are solid (the boundary
RegionMember[reg, {x, y, …}]
RegionMember[reg]
RegionMember[Disk[{0, 0}, 1], {0.5, 0.5}]
→ TrueRegionMember[Disk[{0, 0}, 1], {2, 0}]
→ FalseRegionMember[Rectangle[{0, 0}, {2, 3}], {1, 1}]
→ True