



 
Hi Tront, I think the term you want is logical implication. There are different, equivalent definitions of a rectangle and a square. Here are two of them.
If a plane figure is a square then a logical implication of this is that it is also a rectangle. To prove this implication assume that a plane figure $F$ is a square. Then $F$ is a quadrilateral with 4 sides of equal length and 4 right angles. Hence $F$ is a quadrilateral and it has 4 right angles. Thus $F$ is a square. I hope this helps,  


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