ctg² a(2sin² a+cos² a-1) = ctg² a(2sin² a-(-cos² a+1) =ctg² a(2sin² a - sin² a) =
(Cos² a/sin² a)*sin² a = Cos² a.
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ctg² a(2sin² a+cos² a-1) = ctg² a(2sin² a-(-cos² a+1) =ctg² a(2sin² a - sin² a) =
(Cos² a/sin² a)*sin² a = Cos² a.