How do i prove that a quadrilateral is a rectangle?

I think you want to prove that a rectangle is a quadrilateral. You can prove this with definitions. The definition of a quadrilateral is that it is a shape with four sides. Since a rectangle has four sides, that proves that a rectangle is a quadrilateral. 

There are a few criterias...
One angle is a right angle
Two pairs of opposite sides that are congruent
Congruent diagonals
Opposite sides are parallel...

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