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Euclid.
BOOK I.
PROPOSITION I. PROBLEM.
N a given finite straight line (
) to describe an equilateral triangle.
Describe 
and 
(postulate 3.); draw 
and 
(post. 1.).
then will 
be equilateral.
For = (def. 15.);
and = (def. 15.),
∴ = (axiom. 1.);
and therefore is the equilateral triangle required.
Q.E.D
