.
the length of the side = = = = 10 centimeters.
Since the diagonals of any rhombus bisect each other and are perpendicular.
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See the lessons on rhombis in this site
- Diagonals of a rhombus are perpendicular
- Diagonals of a rhombus bisect its angles
- The length of diagonals of a rhombus
- A circle inscribed in the rhombus
- HOW TO solve problems on the rhombus sides and diagonals measures - Examples
- The length of diagonals of a rhombus
- PROPERTIES OF RHOMBIS
Also, you have this free of charge online textbook on Geometry
GEOMETRY - YOUR ONLINE TEXTBOOK
in this site.
The referred lessons are the part of this textbook under the topic "Properties of rhombis".
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.
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Notation of sides of the rhombus in your post is INCORRECT !