Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$\sin A \cos B+\cos A \sin B$
The sum formula for the sine function is: $$\sin (A+B)=\sin A \cos B+\cos A \sin B$$ Thus, the missing expression in the given statement is: $$\sin A \cos B+\cos A \sin B$$