Diketahui cos a = 3/5, dengan 0° < a < 90°. Tentukan nilai trigonometri tan 2a

cos a = 3/5
sisi samping 3
sisi miring 5
sisi depan 4
tan a = 4/3
tan 2a = 2.tan a / (1 - tan²a)
= 2.(4/3) / (1 - (4/3)²)
= (8/3) / (1 - 16/9)
= (8/3) / (9/9 - 16/9)
= (8/3) / (-7/9)
= (8/3) × (-9/7)
= -24/7