Jawab pake cara No 3

sin α = 4 / 5

maka
sisi depan = 4
sisi miring = 5
sisi samping = √5² - 4² = √25-16 = √9 = 3

cos α = 3/5

sin β = 12/13
maka
sisi depan = 12
sisi miring = 13
sisi samping = √13² - 12² = √169 - 144 = √25 = 5

cos β = 5/13   

cos (α+β) = cos α . cos β - sin α . sin β
                = 3/5 . 5/13 - 4/5 . 12/13
                = 15/65 - 48/65
                = - 33/65

Sin a = 4/5
Cos a = 3/5

Sin β = 12/13
Cos β = 5/13

Maka...
Cos (a + β)
= Cos a Cos β - Sin a Sin β
= (3/5)(5/13) - (4/5)(12/13)
= 3/13 - 48/65
= 15/65 - 48/65
= -33/65