T H E E L E M E N T S	Book II   Proposition 4
	Therefore     the four areas HF, CK, AG, GE are equal to the squares on AC, CB and twice the rectangle contained by AC, CB. But     HF, CK, AG, GE are the whole ADEB, which is the square on AB. Therefore     the square on AB is equal to     the squares on AC, CB and     twice the rectangle contained by AC, CB. Being what it was required to prove.
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