Why is a triangle not a quadrilateral?

Answer 1

A triangle can't be a quadrilateral because quadrilateral means it has to have 4 sides and they have to be equal and a triangle can only have 3 sides and doesn't have to be all equal sides.

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Mainly correct, but the sides of a quadrilateral do not have to be equal.

Answer 2

A triangle, as the prefix tri- suggests, is a polygon with 3 sides.A quadrilateral, as the prefix quad- suggests, is a polygon with 4 sides.

Because 3 is not equal to 4, a triangle cannot possibly be a quadrilateral.