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If the diagonals of a parallelogram are perpendicular, then the parallelogram is a rhombus.
In this case, the diagonals divide the rhombus in four congruent right-angled triangles.
For each such a triangle, the legs are 6/2 = 3 units and 8/2 = 4 units;
hence, the sides of the rhombus are = 5 units long, and its perimeter is 5*4 = 20 units.
Solved.
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On relevant properties of rhombuses, see the lessons
- Diagonals of a rhombus are perpendicular
- Diagonals of a rhombus bisect its angles
- The length of diagonals of a rhombus
- HOW TO solve problems on the rhombus sides and diagonals measures - Examples
in this site.
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.