T H E
E L E M E N T S 
Book I
Proposition 11

Let the point D have been taken at random on AC,
and
let CE have been cut off equal to CD, [I.3]
and
let the equilateral triangle DEF have been constructed upon DE, [I.1]
and
let FC have been joined.
I say that
the straignt line FC has been drawn at right angles to the given straight line AB from C the given point on it.


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