T H E E L E M E N T S |
Book III Proposition 22 | |
But the angles ABC, BAC, ACB are equal to two right angles; therefore the angles ABC, ADC are also equal to two right angles. Similarly we can prove that the angles BAD, DCB are also equal to two right angles. Therefore the opposite angles of quadrilaterals in circles are equal to two right angles. Being what it was required to prove. |
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