What is the value of $cos((2pi)/3)$ ?

Jul 30, 2016

$-1/2$

$(2pi)/3 "is an angle in the " color(blue)"2nd quadrant where the cos ratio is negatve"$

$rArrcos((2pi)/3)=-cos(pi-(2pi)/3)=-cos(pi/3)=-1/2$

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What is the value of cos((2pi)/3)cos((2pi)/3)?