Math-pre-calculus

Given tan theta=-8/5 and sin theta < 0, find cos theta.

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  1. if tanØ < 0 and sinØ < 0 , then Ø must be in quad IV
    from tanØ = -8/5 in IV, we have x = 5, y = -8
    r^2 = 25+64 = 89
    r = √89

    then cosØ = 5/√89

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