Trig math

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how to split up using sum and difference identities

cos (13pi/12)

PLeaseeeeeeeeeee help!!! thanx a million

  • Trig math -

    Sometimes it is easier for students to see these relationships in degrees.
    13π/12 radians = 195°
    and 195 = 45+150

    Knowing the ratio of sides of the 30-60-90 and the 45-45-90 triangles we can find that
    sin 150°=1/2
    cos 150° = -√3/2

    sin 45° = cos 45° = 1/√2 = √2/2

    then back to
    = cos(195)
    = cos(150+45)
    = cos150 cos45 - sin150 sin45
    = (-√3/2)(√2/2) - (1/2)(√2/2
    = (-√6 - √2)/4

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