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'''Mass points''' is a technique in [[Euclidean geometry]] that can greatly simplify the proofs of many theorems concerning [[polygon]]s, and is helpful in solving complex geometry problems involving lengths.  In essence, it involves using a local [[coordinate system]] to identify [[point]]s by the [[ratio]]s into which they divide [[line segment]]s.  Mass points are generalized by [[barycentric coordinates]].

Mass point geometry was invented by Franz Mobius in 1827 along with his theory of homogeneous coordinates. The technique did not catch on until the 1960s when New York high school students made it popular. The technique greatly simplifies certain problems. 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. That is, if two points (<math>A</math> and <math>B</math>) have masses <math>m</math> and <math>n</math>, respectively, a third point between <math>A</math> and <math>B</math> which divides <math>AB</math> into the ratio <math>\tfrac mn</math> will have mass <math>m + n</math>.

== Examples ==
== Example 1==
Consider a triangle <math>ABC</math> with its three [[Median_(geometry)|median]]s drawn, with the intersection points being <math>D, E, F,</math> corresponding to <math>AB, BC,</math> and <math>AC</math> respectively. Thus, if we label point <math>A</math> with a weight of <math>1</math>, <math>B</math> must also have a weight of <math>1</math> since <math>A</math> and <math>B</math> are equidistant from <math>D</math>. By the same process, we find <math>C</math> must also have a weight of 1. Now, since <math>A</math> and <math>B</math> both have a weight of <math>1</math>, <math>D</math> must have a weight of <math>2</math> (as is true for <math>E</math> and <math>F</math>). Thus, if we label the centroid <math>P</math>, we can deduce that <math>DP:PC</math> is <math>1:2</math> - the inverse ratio of their weights.
== Example 2==
<math>\triangle ABC</math> has point <math>D</math> on <math>AB</math>, point <math>E</math> on <math>BC</math>, and point <math>F</math> on <math>AC</math>. <math>AE</math>, <math>CD</math>, and <math>BF</math>  intersect at point <math>G</math>. The ratio <math>AD:DB</math> is <math>3:5</math> and the ratio <math>CE:EB</math> is <math>8:3</math>. Find the ratio of <math>FG:GB</math>

== Solution==

Throughout this solution, let <math>W_{X}</math> denote the weight at point <math>X</math>. Since <math>\frac{CE}{EB} = \frac{8}{3}</math>, let <math>W_{B} = 8</math>, which makes <math>W_{C} = 3</math>. Now, look at <math>AB</math>. Since <math>W_{B} \cdot BD = W_{A} \cdot AD</math> (this is a general property commonly used in many mass points problems, in fact it is the same property we used above to determine <math>W_{B}</math>), we have <math>W_{A} = W_{B} \cdot \frac{BD}{AD} = 8 \cdot \frac{5}{3} = \frac{40}{3}</math>. Then, <math>W_{F} = W_{A} + W_{C} = \frac{40}{3} + 3 = \frac{49}{3}</math> (another property of mass points). Finally, we have <math>FG:GB = W_{B}:W_{F} = 8:\frac{49}{3} = \boxed{24:49}</math>.

==Problems==
[[2019 AMC 8 Problems/Problem 24]]

[[2016 AMC 10A Problems/Problem 19]]

[[2013 AMC 10B Problems/Problem 16]]

[[2004 AMC 10B Problems/Problem 20]]

[[2016 AMC 12A Problems/Problem 12]]

[[2009 AIME I Problems/Problem 5]]

[[2009 AIME I Problems/Problem 4]]

[[2001 AIME I Problems/Problem 7 ]]

[[2011 AIME II Problems/Problem 4]]

[[ 1992 AIME  Problems/Problem 14 ]]

[[ 1988 AIME Problems/Problem 12]]

[[1989 AIME Problems/Problem 15]]

[[1985 AIME Problems/Problem 6]]
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[[1971 AHSME Problems/Problem 26]]

==More Info==
See [https://en.wikipedia.org/wiki/Mass_point_geometry "Mass point geometry" on Wikipedia]
==Video Lectures==

The Central NC Math Group released a lecture on Mass Points and Barycentric Coordinates, which you can view at https://www.youtube.com/watch?v=KQim7-wrwL0.

Here's another one from Double Donut: https://www.youtube.com/watch?v=VtBgp5WZni8



[[Category:Definition]]
[[Category:Geometry]]