Expand Using Sum/Difference Formulas cos((4pi)/21)cos(pi/7)-sin((4pi)/21)sin(pi/7)

$\cos (\frac{4 π}{21}) \cos (\frac{π}{7}) - \sin (\frac{4 π}{21}) \sin (\frac{π}{7})$

Step 1

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$\cos (\frac{4 π}{21}) \cos (\frac{π}{7}) - \sin (\frac{4 π}{21}) \sin (\frac{π}{7})$