Answer: B. sin a
Explanation
• Expression,
[tex]_{---}\cos b=\frac{1}{2}\left[\sin\left(a+b\right)+\sin\left(a-b\right)\right][/tex]
If we use the properties we get that:
[tex]2\sin a\cos b=\left[\sin\left(a+b\right)+\sin\left(a-b\right)\right][/tex]
Then:
[tex]\sin a\cos b=\frac{1}{2}\left[\sin\left(a+b\right)+\sin\left(a-b\right)\right][/tex]
