There must be an error...
I tried to use Pythagoras' theorem: 4² + (1/2(1/5a))² = a² (a = CD)
Solve this for a and you'll get a = (40*sqrt(11)) / 33. Then, DE must be a/5 = (8*sqrt(11)) / 33.
Finally the area *should* be (16 * sqrt(11)) / 33... Boo you died!
Strange...
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