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rhombus
In Euclidean geometry, a rhombus(◊), plural rhombi or rhombuses, is a simple (non-self-intersecting) quadrilateral all of whose four sides have the same length. Another name is equilateral quadrilateral, since equilateral means that all of its sides are equal in length. The rhombus is often called a diamond, after the diamonds suit in playing cards which resembles the projection of an octahedral diamond, or a lozenge, though the former sometimes refers specifically to a rhombus with a 60° angle (see Polyiamond), and the latter sometimes refers specifically to a rhombus with a 45° angle.
Every rhombus is a parallelogram and a kite. A rhombus with right angles is a square.〔Note: Euclid's original definition and some English dictionaries' definition of rhombus excludes squares, but modern mathematicians prefer the inclusive definition.〕〔 inclusive usage〕
==Etymology==
The word "rhombus" comes from Greek (''rhombos''), meaning something that spins,〔(), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus〕 which derives from the verb ρέμβω (''rhembō''), meaning "to turn round and round".〔(ρέμβω ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus〕 The word was used both by Euclid and Archimedes, who used the term "solid rhombus" for two right circular cones sharing a common base.〔(The Origin of Rhombus )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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