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Postulate 4

That all right angles equal one another.

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| In the definition of right angle, it is clear that the two angles at the foot of a perpendicular, such as angles ACD and BCD, are equal. This postulate says that an angle at the foot of one perpendicular, such as angle ACD, equals an angle at the foot of any other perpendicular, such as angle EGH.

Book I Introduction - Postulate 3 - Postulate 5.

© 1996
D.E.Joyce
Clark University