Question

how quadrilateral is said to be unique

Answer 1

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In a parallelogram, opposite sides are equal, opposite angles are equal and diagonals bisect each other. In a rhombus diagonals intersect at right angles. In a rectangle diagonals are equal. A quadrilateral can be constructed uniquely if its two adjacent sides and three angles are given.

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Answer 2

A quadrilateral is a polygon with four edges and four vertices

.The unique feature about a quadilateral is that has  a single angle measuring more than 180.

If five measurements are given a quadilateral can be constucted.

Like if four lengths and one diagonal is given. or three sides and two diagonals are given etc.