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Math Help - Radians and surds

  1. #1
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    Radians and surds

    Having problems with working out surds from radians. Does anyone now the whereabouts on the net where there is an extensive chart? At the moment i am struggling with cos(5*pi/6).
    Cheers in advance
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  2. #2
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    Quote Originally Posted by kendon
    Having problems with working out surds from radians. Does anyone now the whereabouts on the net where there is an extensive chart? At the moment i am struggling with cos(5*pi/6).
    Cheers in advance
    You got a calculator? Good.

    Your problem is this, you need a way to express trigonometric functions of certain angles in radical form. I remember when I was in 11th grade my cl*** was going insane with this so I have a trick which you can do.
    For example,
    \cos \frac{5\pi}{6}=-0.86602541
    Now, if you end up with a long decimal square,
    (-0.86602541)^2=.75
    But,
    .75=\frac{3}{4}
    Now, that the square root back.
    \sqrt{\frac{3}{4}}=\frac{\sqrt{3}}{2}
    Important. When you square the sign always goes away. Therefore, if you had a sign in the beginning (and you did) when you need to attach a sign here.
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  3. #3
    Grand Panjandrum
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    Quote Originally Posted by kendon
    Having problems with working out surds from radians. Does anyone now the whereabouts on the net where there is an extensive chart? At the moment i am struggling with cos(5*pi/6).
    Cheers in advance
    An angle of 5 \pi/6=-\pi/6, now \pi/6 is 30 degrees, and so its cosine is \sqrt{3}/2 (consider the sides
    of half of an equilateral triangle), and the cosine function is even so

    \cos(5 \pi/6)=\cos(-\pi/6)=\cos(\pi/6)=\sqrt{3}/2

    RonL
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