2007 AIME I Problems/Problem 4
Three planets orbit a star circularly in the same plane. Each moves in the same direction and moves at constant speed. Their periods are 60, 84, and 140. The three planets and the star are currently collinear. What is the fewest number of years from now that they will all be collinear again?
Denote the planets respectively. Let denote the angle which each of the respective planets makes with its initial position after years. These are given by , , .
In order for the planets and the central star to be collinear, , , and must differ by a multiple of . Note that and , so . These are simultaneously multiples of exactly when is a multiple of , so the planets and the star will next be collinear in years.
This problem is trivialized since the answer is the LCM/GCF of . This is just which is -Brudder
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