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Richard
Lv 4
Richard asked in Science & MathematicsMathematics · 1 decade ago

What is the exact value of cos(21pi/12)?

cos(x+y) or cos(x-y)

3 Answers

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  • Tam
    Lv 6
    1 decade ago
    Favorite Answer

    cos(21π/12)

    =cos(7π/4)

    = cos(3π/4 + π)

    Use the cos(a+b) identity: cos(a+b)=cos(a)cos(b)-sin(a)sin(b)

    = cos(3π/4)cos(π) - sin(3π/4)sin(π)

    = -1/√(2) * (-1) - 1/√(2) * (0)

    = 1/√(2)

  • iceman
    Lv 7
    1 decade ago

    cos(21pi/12)

    = cos(7pi/4)

    = cos(pi/4)

    = (sqrt2)/2

  • not mentions
    1 decade ago

    pi...cosign is TT. pi=3.14 There is actually the pi symbol under "symbols" in Word...LITERALLY.

    I remember this from integrated math classes back in high school. If you're talked 21pi as in 21 * star means multiply, 3.14 * 21 = 65.94 65.94 / 12 = 5.49

    Hint: Always do parenthesis first.

    Source(s): self
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