T H E E L E M E N T S	Book I   Proposition 33
	And, since AB is equal to CD, and BC is common,     the two sides AB, BC are equal to the two sides DC, CB; and the angle ABC is equal to the angle BCD; therefore     the base AC is equal to the base BD; and the triangle ABC is equal to the triangle BCD, and the remaining angles will be equal to the remaining angles restectively, namely those which the equal sides subtend; [I.4] therefore     the angle ABC is equal to the angle CBD.
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