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GBCA is equal to DEFH; for they are on equal bases BC, EF and in the same parallels BF, GH. [I.36]
Moreover
the triangle ABC is half of the parallelogram GBCA; for the diameter AB bisects it. [I.34]
And
the triangle DEF is half of the parallelogram DEFH; for the diameter DF bisects it. [I.34]
