Answer
The given statement makes sense.
Work Step by Step
Once the formula for differences of cosines is verified, the other sum and difference formulas can be verified just by changing the values of angles.
Let us take an example,
$\cos \left( a-b \right)=\cos a\cos b+\sin a\sin b$
Replace b by -b:
$\cos \left( a+b \right)=\cos a\cos b-\sin a\sin b$
Hence, verified.
