tan a= 3/4 dan tan b=1/7. nilai cos (a+b)

diketahui 
tan a = 3/4
maka
sin a = 3/5
cos a = 4/5

tan b = 1/7
--------------------------------
7² + 1² = 50 = (5√2)²
----------------------------------
maka
sin b = (1/10)√2
cos b = (7/10)√2

jadi 
cos(a + b)
= (cos a . cos b) - (sin a . sin b)
= ([4/5] [(7/10)√2]) - ([3/5] [(1/10)√2])
= (28/50)√2 - (3/50)√2
= (25/50)√2
= (1/5)√2