Buktikan (2cosa-1)(2cosa+1) = 2cos2a+1

jawab
cos 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