Prove that the sum of the squares of the diagonals of parallelogram is equal to the sum of the squares of its sides.
In ABCD, let d1 and d2 be the diagonals AC and BD, respectively. Construct a coordinate system so that A is the origin, B lies on the positive x axis, and C and D lies above the x axis. See the diagram below. Assume that AB is equal to DC and AD is equal to BC. We must prove that 
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Let x be the x coordinate of B, let C have coordinates (x + x0, y) and D have the coordinates (x0, y). If x0 is 0 then the parallelogram is a rectangle.
Firstly, using the distance formula, let's solve for the length of the two diagonals, d1 and d2.
Now let's sum the squares of the diagonals:
Secondly, again using the distance formula, let's solve for the length of the two sides AB and AD.
Finally, we want to sum the squares of the sides and multiply by 2 (4 sides total, 2 of each length).
Thus, we have proven that the sum of the squares of the four sides of a parallelogram is equal to the sum of the squares of the diagonals.
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