trig

find cos (x/2) when sint =12/13

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  1. sint = 12/13
    then cost = 5/13 , by Pythagoras.

    you probably meant find cos (t/2)

    using cos 2A = 2cos^2 A - 1
    cost = 2cos^2 (t/2) - 1
    18/13 = 2cos^2 (t/2)
    9/13 = cos^2 (t/2)
    cos (t/2) = 3/sqrt(13)

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