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Lê Quốc Trần Anh Coordinator

14/03/2018 at 09:41
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On a standard 12-hour clock, the minute hand moves continuously, at a constant rate, making one full revolution every hour, and the hour hand moves similarly, making one full revolution every 12 hours. What is the measure of the smaller of the two angles between the minute hand and the hour hand, in degrees, when the clock reads 5:42?




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    Lê Anh Duy 15/03/2018 at 06:05

         At 5:42, the time elapsed is 42 of the 60 minutes in the 5 o'clock  hour. Since the minute hand will make the compele revolution during that hour, at 5:42 it has traveled \(\dfrac{42}{60}=\dfrac{7}{10}\) of the full 360 degrees, or\(\dfrac{7}{10}.360^o=252^o\).

         The hour hand makes \(\dfrac{1}{12}\) of a complete revolution every hour. So, from 12:00 to 5:00, it travels \(\dfrac{5}{12}\) of the full 360 degrees, or \(\dfrac{5}{12}.360^o=150^o\)

         By 5:42, it has traveled another \(\dfrac{7}{10}\) of \(\dfrac{1}{12}\) of the full degrees, or \(\dfrac{7}{10}.\dfrac{1}{12}=21^o\), for a total of 150 + 21 = 171 degrees.

         The measure of the angle between the hands at 5;42 is 252 - 171 = 81 degree

    \(ANSWER:81^o\)

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