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Math Help - Trick to solve ??

  1. #1
    F_S
    F_S is offline
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    Trick to solve ??

    What is the trick to solve these ?

    1. cos (2pi/7) + cos (4pi/7) + cos (6pi/7) = ?

    2. sin (pi/10) * sin (13pi/10) = ?

    I believe there has to be a simple trick to solve these, but am unable to figure that so far : ( .

    Any clue ?
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  2. #2
    Member Greengoblin's Avatar
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    Looks like you should plug it into a calculator from here. Most of the co-efficients of \pi aren't obvious ones such as \frac{\pi}{2}, \frac{\pi}{3}, \frac{3\pi}{2}, and only one of them is top-heavy ( \frac{13}{10}), which can be simplified to \frac{3}{10}, which again is not a very obvious one. There's no algebraic tricks as far as I can see.
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