Use an addition or subtraction formula to write the expression as a trigonometric function of one number:
$\cos \frac{3\pi}{7} \cos \frac{2\pi}{21} + \sin \frac{3\pi}{7} \sin \frac{2\pi}{21} =\cos \frac{\pi}{A} =\frac{B}{2}$.
$A=$ ,
$B=$ .