Algebra 2/ Trigonometry

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If cos y = -4/5 when 90°≤ y ≤ 180° what is the value of cos 2y?

  • Algebra 2/ Trigonometry -

    quadrant 2

    cos 2y = 1 - 2 sin^2 y
    but sin y = 3/5 (3 4 5 triangle)

    cos 2 y = 1 - 2 (3/5)
    = 1 - 6/5
    = -1/5

  • Algebra 2/ Trigonometry -

    last 3 lines should be

    cos (2y) = 1 - 2(3/5)^2
    = 1 - 18/25 = 7/25 or .28

  • Algebra 2/ Trigonometry -

    Whoops, thank you

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