What is the value of cos((4pi)/7)cos((5pi}{21})+sin((4pi)/7)sin(\frac{5pi)/(21)) ?

The value of cos((4pi)/7)cos((5pi}{21})+sin((4pi)/7)sin(\frac{5pi)/(21)) is 1/2

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cos((4pi)/7)cos((5pi}{21})+sin((4pi)/7)sin(\frac{5pi)/(21))

cos (\frac{4 π}{7}) cos (\frac{5 π}{21}) + sin (\frac{4 π}{7}) sin (\frac{5 π}{21})

\frac{1}{2}

Solution steps

cos (\frac{4 π}{7}) cos (\frac{5 π}{21}) + sin (\frac{4 π}{7}) sin (\frac{5 π}{21})

Rewrite using trig identities:

cos (\frac{π}{3})

= cos (\frac{π}{3})

Use the following trivial identity:

cos (\frac{π}{3}) = \frac{1}{2}

= \frac{1}{2}

arctan (cos (\frac{3 π}{2}))

9.8 \cdot cos (3 0^{\circ})

sin (arccos (0.6))

- sin^{2} (\frac{π}{4})

tan (π) + sin (π)

What is the value of cos((4pi)/7)cos((5pi}{21})+sin((4pi)/7)sin(\frac{5pi)/(21)) ?
The value of cos((4pi)/7)cos((5pi}{21})+sin((4pi)/7)sin(\frac{5pi)/(21)) is 1/2