Question

cos 20 + cos 40 + cos 60 +
....cos 180

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Answer 1

Answer: cos20 = cos(180-160) = -cos160

cos40 = cos(180-140)= -cos140

cos60 = cos(180-120)= -cos120

cos80 =cos(180-100) = -cos100

now cos20+cos40+cos60+cos80+cos100+cos120+cos140+cos160+cosSubstitutestitue The Above Values

-cos160-cos140-cos120-cos100+cos100+cos120+cos140+cos160+cos180

After Cancelling The Terms U Will Left Out With cos180=-1