No 3 jawab pake cara

sin a = 4/5
cos^2 a = 1 - sin^2 a = 1 - (4/5)^2 = 1 - 16/25 = 9/25
cos a = √(9/25) = 3/5

sin b = 12/13
cos^2 b = 1 - sin^2 b = 1 - (12/13)^2 = 1 - (144/169) = 25/169
Cos b = √(25/169) = 5/13

Cos (a + b) = cos a cos b - sin a sin b
= 3/5 . 5/13 - 4/5 . 12/13
= 15/65 - 48/65
= -33/65