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Trig

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How do i find the exact value of sin 15 using the half angle formula

  • Trig -

    sin (30/2) = +/- sqrt [(1-cos 30)/2]
    + in first quadrant

    = sqrt [ (1 - {sqrt 3}/2)/2) ]

    = sqrt [ (2 - sqrt 3)/4 ]

    = (1/2) sqrt (2 - sqrt 3)

  • Trig -

    use
    cos 2A = 1 - 2sin^2 A
    cos 30° = 1 - 2sin^2 15°
    √3/2 = 1- 2sin^2 15°
    2sin^2 15° - 1 - √3/2 = (2-√3)/2
    sin^2 15° = (2-√3)/4

    sin 15° = √(2-√3) /2

    it would have been easier to do
    sin15°
    = sin(45°-30°)
    = sin45cos30 - cos45sin30
    = (√2/2)(√3/2) -(√2/2)(1/2)
    = (√6 - √2)/4 which yields the same result as above

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