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BOOK I. PROP. XVII. THEOR.
17
NY two angles of a triangle 
are together less than two right angles.
Produce 
, then will
+ 
= 
But, > 
(pr. 16.)
∴ + < ,
and in the same manner it may be shown that any other two angles of the triangle taken together are less than two right angles.
Q.E.D.
