Evaluating $\cos\frac{17\pi}{18} \cos\frac{7\pi}{9}+ \sin\frac{17\pi}{18} \sin\frac{7\pi}{9}$ with a sum or difference formula 
Use a sum or difference formula to find the exact value of the following:
$$\cos\frac{17\pi}{18} \cos\frac{7\pi}{9}+ \sin\frac{17\pi}{18} \sin\frac{7\pi}{9}$$

The answer I got for this was $0.8660254$. Please let me know if my answer is correct.
Any help is greatly appreciated.

Yes; using the difference formula, it's $\cos\left(\dfrac{17\pi}{18}-\dfrac{14\pi}{18}\right)=\cos
\left(\dfrac\pi6\right)=\dfrac{\sqrt3}2.$
