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2020 AMC 10A Problem 12

Problem 12 of 25IntermediateGeometry

Triangle AMCAMC is isosceles with AM=AC.AM = AC. Medians MV\overline{MV} and CU\overline{CU} are perpendicular to each other, and MV=CU=12.MV=CU=12. What is the area of AMC?\triangle AMC?

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Solution

Let the centroid be the origin. Since a centroid divides each median in a 2:12:1 ratio, we may place median MVMV horizontally with M=(8,0)M=(8,0) and V=(4,0)V=(-4,0), and median CUCU vertically with C=(0,8)C=(0,8) and U=(0,4)U=(0,-4). Because UU is the midpoint of AMAM, we get A=2UM=(8,8)A=2U-M=(-8,-8). The area of AMC\triangle AMC is 12(16,8)×(8,16)\dfrac12 |(16,8)\times(8,16)| =12(25664)=96=\dfrac12(256-64)=96. Thus, C is the correct answer.

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Concepts: median (geometry) · centroid · coordinate geometry

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.