1989 AIME Problems/Problem 12
Call the midpoint of and the midpoint of . is the median of triangle . The formula for the length of a median is , where , , and are the side lengths of triangle, and is the side that is bisected by median . The formula is a direct result of the Law of Cosines applied twice with the angles formed by the median (Stewart's Theorem). We can also get this formula from the parallelogram law, that the sum of the squares of the diagonals is equal to the squares of the sides of a parallelogram (https://en.wikipedia.org/wiki/Parallelogram_law).
We first find , which is the median of .
Now we must find , which is the median of .
Now that we know the sides of , we proceed to find the length of .
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