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Given cot theta = 5, find sec theta

 Jan 22, 2017
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Since COT = +5    and COT = cos / sin     we know that theta must be in  1st and 3rd quadrants

so    cos/sin = 5  (this was given)

        cos = 5 sin      Now square both sides

        cos^2 = 25 sin^2   (1st equation)

 

sin^2 + cos^2 = 1       so sin^2 = 1-cos^2      Sub this into 1st equation

               cos^2 = 25(1-cos^2)

               cos^2 = 25 - 25 cos^2      subtract cos^2 from both sides

                 0   =  25 - 26 cos^2          Subtract   25 from both sides

    -25 = - 26 cos^2

      25 = 26 cos^2

      25/26 = cos^2

    +-  5/ (sqrt 26 ) = cos theta        

   Sec theta = 1/cos   =   1/ (+- 5/sqrt26) =  sqrt 26 / 5  or   - sqrt26 / 5

 Jan 23, 2017
edited by ElectricPavlov  Jan 23, 2017

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