Answer:
56/65
Step-by-step explanation:
First, we know that cos(a-b) = cos(a)cos(b) + sin(a)sin(b)
We know what sin(a) and sin(b) are, and to get cos(a), we can take the equation sin²a + cos²a = 1
Thus,
(12/13)² + cos²a = 1
1 - (12/13)² = cos²a
1- 144/169 = cos²a
cos²a = 25/169
cos(a) = 5/13
Similarly,
(3/5)² + cos²b = 1
1 - (3/5)² = cos²b
1 - 9/25 = cos²b
cos²b = 16/25
cos(b) = 4/5
Our answer is
cos(a-b) = cos(a)cos(b) + sin(a)sin(b)
cos(a-b) = (5/13)(4/5) + (12/13)(3/5)
cos(a-b) = 20/65 + 36/65
cos(a-b) = 56/65
