There have been two typos. Many thanks for bringing this up.

Area(AMB'C') = Area(MAC') + Area(MBC')

should have been

Area(AMB'C') = Area(MAC') + Area(MB'C')

and

Area(AMB'C') = Area(AMB') + Area(ABC')

should have been

Area(AMB'C') = Area(AMB') + Area(AB'C')

Both are fixed now.