T H E E L E M E N T S |
Book IV Proposition 10 | |
But the angle BDA is equal to the angle CBD, since the side AD is also equal to AB; [I.5] so that the angle DBA is also equal to the angle BCD. Therefore the three angles BDA, DBA, BCD are equal to one another. And, since the angle DBC is equal to the angle BCD, the side BD is also equal to the side DC. [I.6] But BD is by hypothesis equal to CA; therefore CA is also equal to CD, so that the angle CDA is also equal to the angle DAC. |
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