T H E E L E M E N T S	Book IV   Proposition 11
	Then, since each of the angles ACD, CDA is double of the angle CAD, and they have been bisected by the straight lines CE, DB, therefore     the five angles DAC, ACE, ECD, CDB, BDA are equal to one another. But equal angles stand on equal circumferences; [III.26] therefore     the five circumferences AB, BC, CD, DE, EA are equal to one another. But equal circumferences are subtended by equal straight lines; [III.29] therefore     the five straight lines AB, BC, CD, DE, EA are equal to one another; therefore     the pentagon ABCDE is equilateral.
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