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Verified answer
Jawabcos 2a = 2 cos² a - 1
2 cos² a = cos 2a + 1
(2 cos a - 1 )(2 cos a + 1) = 2 cos (2a) + 1
(2cos a - 1)(2cos a + 1) =
= 4 cos² a - 1
= 2 ( 2 cos² a ) - 1
= 2 ( cos 2a + 1) - 1
= 2 cos 2a + 2 - 1
= 2 cos 2a + 1