Jika tan2x = 4 sinxcosx maka cos x=?

jawab
tan 2x = 4 sin x cos x
sin 2x/cos 2x = 2 ( 2 sin x cos x)
sin 2x/cos 2x  = 2 (sin 2x)
sin 2x = 2 sin 2x cos 2x
1 = 2 cos 2x
cos 2x = 1/2 

cos 2x = 2 cos² x -1

2cos² x - 1 =12
2 cos² x = 1/2 + 1 = 3/2
cos² x = 3/4
cos x  = +_√ (3/4) = +_1/2 √3  
cos x = 1/2 √3   atau cos x = - 1/2 √3