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BOOK I. PROP. IX. PROP.
9
O bisect a given rectilinear angle (
).
Take 
= (pr. 3.) draw 
, upon which describe 
(pr. 1.), draw 
.
Because = (const.) and common to the two triangles and 
= (const.),
∴ 
= 
(pr. 8.)
Q.E.D.
