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BOOK I. PROP. XI. PROB.
11
ROM a given point (
), in a given straight line (), to draw a perpendicular.
Take any point (
) in the given line, cut off 
= 
(pr. 3.), construct 
(pr. 1.), draw 
and it shall be perpendicular to the given line.
For 
= (const.)
= (const.)
and common to the two triangles.
Therefore 
= 
(pr. 8.)
∴ (def. 10.).
Q.E.D.
