Jika 0 < a,b < 90° ,sin a =4/5 ,dan sin (a+b) =12/13 ,maka cos b=

sin a = 4/5 = de/mi => sa = √(5^2 - 4^2) = √9 = 3
cos a = sa/mi = 3/5

sin (a + b) = 12/13 = de/mi => sa = √(13^2 - 12^2) = √25 = 5
cos (a + b) = sa/mi = 5/13

sin (a + b) = sin a cos b + cos a sin b => 12/13 = 4/5 cos b + 3/5 sin b |x4|
cos (a + b) = cos a cos b - sin a sin b => 5/13 = 3/5 cos b - 4/5 sin b |x3|

16/5 cos b + 12/5 sin b = 48/13
9/5 cos b - 12/5 sin b = 15/13
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25/5 cos b = 63/13
5 cos b = 63/13
cos b = 63/65