Euclid's
Elements
 Book 1: The 48 Propositions
| 5 |
In
isosceles triangles the angles at the base are equal to one another, and if the
equal straight lines be produced further, the angles under the base will be
equal to one another. |
| Let ABC be
an isosceles triangle having the side AB equal to the side AC; and let the
straight lines BD, CE be produced further in a straight line with AB,
AC. |
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