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Parallelogram Diagonals

Use coordinate geometry to prove that the diagonals of a parallelogram bisect each other. That is, write a coordinate geometry proof that formally proves what this applet informally illustrates. Be sure to assign appropriate variable coordinates to your parallelogram's vertices! (The maximum number of variable coordinates your coordinate setup should have is 3.)
Now prove this same theorem true using the format of a 2-column proof or a paragraph proof. Hint: This proof may contain multiple pairs of congruent triangles. In your opinion, was this theorem easier to prove using a coordinate geometry setup OR a paragraph (or 2-column) proof setup? What do you think?