Evaluate cos (2 pi/3)

we have to find the value of cos(2pi/3)

Solution

We know that pi=180 degrees

cos(2pi/3)= cos (2 X 180/3)= cos 120

We can find cos 120 using cos 90 and cos 180

Cos 120° = cos(180° – 60°) = – cos 60° = -½ (since cos(180° – x) = – cos x)

Cos 120° = cos(90° + 30°) = – sin 30° = -½ (we know that cos (90° + x) = -sin x)

Answer

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

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