.
It is a classic Elementary Geometry theorem:
If in a triangle two sides are unequal, then the angle opposite to the longer side is greater than the angle opposite to the shorter side.
For the proof see the lesson
- Angles and sides inequality theorems for triangles, Theorem 1
in this site.
Also, you have this free of charge online textbook on Geometry
- GEOMETRY - YOUR ONLINE TEXTBOOK
in this site.
The referred lesson is the part of this textbook in the section "Properties of triangles".