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It is given that A is an obtuse angle and cosA=-sqrt5/3.

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Without using calculator,find the value of cos 3A

 

 
asked Jul 14, 2013 in TRIGONOMETRY by anonymous Apprentice
  

1 Answer

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cosA=-sqrt5/3.

Using formula cos3A=4cos^3A-3cosA

Substitute -sqrt5/3 for cos A

cos3A=4(-sqrt5/3)^3-3(-sqrt5/3)

            =-4(5sqrt5/3)+3sqrt5/3

             =-20sqrt5/3+3sqrt5/3

             =-17sqrt5/3

answered Jul 14, 2013 by bradely Apprentice

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