Question

Parallelogram ABCD has the following vertices: A(3,6), B(6,8), C(9,6), and D(6,4). In two ut more complete sentences, explain how you can use the coordinates of the vertices to prove that parallelogram ABCD is a rhombus.​

Answer 1

Explanation:

A rhombus is a parallelogram in which the diagonals are mutual perpendicular bisectors. The coordinates of the vertices can be used to show ...

• the diagonals bisect each other
• the diagonals are perpendicular

When both these conditions are met, the quadrilateral is a rhombus.

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Mutual Bisectors

The diagonals will have the same midpoint (bisect each other) if the sum of the coordinates at the ends of one diagonal is the same as the sum of the end point coordinates of the other diagonal:

  A +C = B +D

  (3, 6) +(9, 6) = (6, 8) +(6, 4)

  (3+9, 6 +6) = (6 +6, 8 +4)

  (12, 12) = (12, 12) . . . . . . midpoints are the same; ABCD is a parallelogram

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Perpendicular

The diagonals will be perpendicular if the sum of the products of the coordinate differences of their end points is zero.

  AC⋅BD = (C -A)⋅(D -B) = (9 -3, 6 -6)⋅(6 -6, 4 -8)

  = (6, 0)⋅(0, -4) . . . . . end point differences; we want their dot-product

  = (6)(0) +(0)(-4) = 0 +0 = 0 . . . . diagonals are perpendicular

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Additional comment

The midpoints are computed as (A+C)/2 and (B+D)/2. We have multiplied each of them by 2 to simplify the arithmetic. Our resulting sums of (12, 12) must be divided by 2 to get the actual midpoint of the rhombus: (6, 6).

The notion of "dot product" is a vector notion. Two vectors are perpendicular when their dot product is zero. The dot product is the scalar sum of the products of corresponding coordinates.