Respuesta :

HomertheGenius
cos 2 X = 1/3
cos 2 X = cos ² X - sin² X
cos ² X - sin ² X = 1/3
+
cos ² X + sin ² X = 1
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2 cos ² X = 4/3    / : 2
cos ² X = 2/3  
cos X = √2/3  ( angle X is in the 1st quadrant )