Jika 2 cos (Φ+π/4) = cos (Φ-π/4), maka tan 2¤ adalah

jawab
π/4 = 180/4 = 45
2 cos(x + 45) = cos (x - 45)

2 ( cos x cos 45 - sin x sin 45) = cos x cos 45 + sin x sin 45
2 (¹/₂√2 cos x - ¹/₂√2 sin x ) =  ¹/₂√2 cos x +  ¹/₂√2 sin x
√2 cos x - √2 sin x = ¹/₂ √2 cos x  + ¹/₂ √2 sin x
- √2 sin x - ¹/₂ √2 sin x = ¹/₂ √2 cos x - √2 cos x
-³/₂ √2  sin x  = - ¹/₂√2 cos x

sin x / cos x = (- ¹/₂√2)/(-³/₂ √2)
tan x = ¹/₃

tan 2x =  (2 tan x) / (1 - tan² x)
= 2(1/3)/ ( 1 - 1/9)
= (2/3) / (8/9)
= 2/3x 9/8
= 3/4