Mass points is a technique in Euclidean geometry that can greatly simplify the proofs of many theorems concerning polygons, and is helpful in solving complex geometry problems involving lengths. In essence, it involves using a local coordinate system to identify points by the ratios into which they divide line segments. Mass points are generalized by barycentric coordinates.
Mass point geometry involves systematically assigning 'weights' to points using ratios of lengths relating vertices, which can then be used to deduce other lengths, using the fact that the lengths must be inversely proportional to their weight (just like a balanced lever). Additionally, the point dividing the line has a mass equal to the sum of the weights on either end of the line (like the fulcrum of a lever).
The way to systematically assign weights to the points involves first choosing a point for the entire figure to balance around. From there, WLOG a first weight can be assigned. From the first weight, others can be derived using a few simple rules. Any line passing this central point will balance the figure. If two points balance, the product of the mass and distance from a line of balance of one point will equal the product of the mass and distance from the same line of balance of the other point. If two points are balanced, the point on the balancing line used to balance them has a mass of the sum of the masses of the two points.
For a full explanation with images, go to this link: http://old.mathtalentquest.org/images/Mass_Point.pdf
Consider a triangle with its three medians drawn, with the intersection points being corresponding to and respectively. Thus, if we label point with a weight of , must also have a weight of since and are equidistant from . By the same process, we find must also have a weight of 1. Now, since and both have a weight of , must have a weight of (as is true for and ). Thus, if we label the centroid , we can deduce that is - the inverse ratio of their weights.
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