Algebra Examples

$\cos (\frac{3 π}{7}) \cos (\frac{2 π}{21}) + \sin (\frac{3 π}{7}) \sin (\frac{2 π}{21})$

Step 1

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$\cos (\frac{3 π}{7}) \cos (\frac{2 π}{21}) + \sin (\frac{3 π}{7}) \sin (\frac{2 π}{21})$