Question

prove that cos20°—cos40°—cos80° =0

Answer 1

_(cos40 +cos 80)+cos 20.[cos(40+80)divides by 2cos(80_40)divides by 2]. =_(cos60degree+cos20degree ) +cos 20degree=cos20degree_cos 20degree _cos 60degree=_cos 60degree =_1divides by 2.

Answer 2

Answer:

cos20°-cos40°-cos80°=0

-(cos 40°+cos 80°+cos20°)

-[(2cos(40+80/2)*cos(40-80/2)]+cos 20°

-[(2cos(120/2)*cos(-40/2)]+cos 20°)

-[(2cos 60°*cos(-20)]+cos20)

-(2*1/2*cos 20°)+ cos 20° {therefore cos(-theta) =cos theta) }

-cos 20°+cos 20°=0

LHS=RHS