Thus the sum of angles BCA and CAB is less than two right angles.
Similarly it is shown that the sum of any two interiors angles is less than two right angles, which was to be proved.
QED
Extend (produce) the side BC to BD.
We know that angle CAB is less than angle CDA.
(Prop I-16)
Add to both angle BCA. The sum of the angles CAB and BCA
is then less than the sum of angles CDA and BCA. However, the
sum of angles CDA and BCA is two right angles.
(Prop I-13)
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