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BOOK I. PROP. XXI. THEOR.
21
F from any point (
) within a triangle 
straight lines be drawn to the extremities of one side (
), these lines must be together less than the other two sides, but must contain a greater angle.
Produce 
,
+ 
> 
(pr. 20.),
add 
to each,
+ 
> + (ax. 4.)
In the same manner it may be shown that
+ > + 
, ∴
+ > + ,
which was to be proved.
Again 
> 
(pr. 16.),
and also 
> (pr. 16.),
∴ > .
Q.E.D.
