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How do you figure out

  1. Jul 6, 2005 #1
    Hello All

    I have this question that I have to find the exact value of sin 5Pi/12. How do I figure out what this is in degrees? Or how do I break this down so I can use addition or subtraction foumulas?

  2. jcsd
  3. Jul 6, 2005 #2


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    How to figure out what 5pi/12 is in degrees? Are you learning from a textbook or teacher, or are you learning on your own? I have to assume that you know how to answer this question. What do you know so far about radians?

    sin(10pi/12) = 2sin(5pi/12)cos(5pi/12) = 2sin(5p/12)SQRT[1 - sin(5pi/12)]

    sin(10pi/12)/2 = sin(5p/12)SQRT[1 - sin(5pi/12)]

    0.5/2 = sin(5p/12)SQRT[1 - sin(5pi/12)]

    You should know that sin(10pi/12) = sin(5pi/6) = sin(pi/6) = 0.5

    1/16 = sin²(5p/12) - sin³(5pi/12)

    16x³ - 16x² + 1 = 0, where x = sin(5pi/12)

    There's a way to factor cubics, but I don't remember it. However, if you look it up on the internet, or even search these forums, you can easily find a method. Once you've factored it, the roots of the equation will naturally be your answer. Note that you might get up to 3 roots, but should only get one that is in [0, 0.5]. Can you tell why the root in that interval will be your answer?
  4. Jul 6, 2005 #3


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    5pi/12=75 degrees
    pi=180 degrees
    What exact values do you know? What identities do you know? The standard exact values to know are pi/10 and pi/12 if you knew them and multiples you would know 5pi/12. you could use the half angle identity to write sin(5pi/12) in terms of cos(5pi/6). You can learn the values for pi/6 and pi/3 by noting pi/3+pi/6=pi/2 and sin(pi/2)=1 cos(pi/2)=0. Also 5pi/6=pi-pi/6. Also don't try to factor a cubic, it would work, but it is difficult and unnecisary.
    Last edited: Jul 6, 2005
  5. Jul 6, 2005 #4
    Thanks for the responses

    I am going to use pi/4 and pi/6 and plug it into SIN(pi/4 + pi/6). This should give me the exatct value. The answer I get is (SQRT(6) + SQRT(2)) /4

    Is this correct?
  6. Jul 6, 2005 #5


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    that is right
  7. Jul 6, 2005 #6
    Thanks. My problem is thinking which combnations would add up to 5pi/12
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