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anonymous

  • one year ago

Find an exact value. cos(pi/12)

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  1. welshfella
    • one year ago
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    i think we can use cos 2x = 2cos^2x - 1 cos (pi/6) = 2 cos^ (pi/12) - 1 cos pi/6 = sqrt3/2

  2. anonymous
    • one year ago
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    they want you to use the half angle formula \[\cos \frac{ x }{ 2 }= \pm \sqrt{\frac{ 1+cosx }{ 2 }}\]

  3. welshfella
    • one year ago
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    oh ok well just plug cos x ( in this case cos pi/6) into the firmula and work it out

  4. anonymous
    • one year ago
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    when I do I don't get the right answer

  5. anonymous
    • one year ago
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    The answer options are (sqrt6+sqrt2)/4 (-sqrt6+sqrt2)/4 (sqrt6-sqrt2)/4 (-sqrt6-sqrt2)/4

  6. welshfella
    • one year ago
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    yes its an awkward one to work out

  7. anonymous
    • one year ago
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    How would you work it out?

  8. welshfella
    • one year ago
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    = sqrt [( 1 + sqrt3/2)/ 2]

  9. anonymous
    • one year ago
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    How do you get from that to one of the answers? I dont understand how you get a sqrt 6 out of any of that

  10. welshfella
    • one year ago
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    i simplified that to (1/2)( sqrt(2 + sqrt3))

  11. welshfella
    • one year ago
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    i agree I think those answers are wrong.

  12. welshfella
    • one year ago
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    because if you go on the calculator cos pi/12 = 0.9659 and my answers comes to the same value

  13. welshfella
    • one year ago
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    oh hold on (sqrt6+sqrt2)/4 comes to 0.9659 as well so that one is right

  14. welshfella
    • one year ago
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    - also the negative of that is another solution

  15. welshfella
    • one year ago
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    I'm not sure how they got to that ...

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