March 29, 2017

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Find the exact value by using an appropriate sum or difference identity.

cos(165) degrees

  • Algebra 2 Honors - ,

    For these type of questions, you must split up your given angles into a sum or a difference of angles whose trig ratios you know

    e.g. 165 = 120+45
    then cos(165)
    = cos(120+45)
    = cos120cos45 - sin120sin45

    you should know both sin45 and cos45
    for sin120,
    sin120 = sin60
    cos120 = -cos60

    take it from here

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