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In a normal geometric language this statements says:
If in a quadrilateral the opposite sides are congruent, then the opposite angles are congruent.
If you know the basic facts of Geometry, then the proof is in two steps:
1. If in a quadrilateral the opposite sides are congruent, then the quadrilateral is a parallelogram.
For the proof, see the lesson
Properties of the sides of a parallelogram, Theorem 2
in this site.
2. In a parallelogram, the opposite angles are congruent.
For the proof, see the lesson
Opposite angles of a parallelogram, Theorem 1
in this site.
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In this site, you have this free of charge online textbook on Geometry
GEOMETRY - YOUR ONLINE TEXTBOOK
The referred lessons are the part of this online textbook under the topic "Properties of parallelograms".
Save the link to this online textbook together with its description
Free of charge online textbook in GEOMETRY
https://www.algebra.com/algebra/homework/Triangles/GEOMETRY-your-online-textbook.lesson
to your archive and use it when it is needed.