1.

20 ^ { \circ } + \operatorname { cos } 100 ^ { \circ } + \operatorname { cos } 140 ^ { \circ } = 0

Answer»

Cos20+cos100=2cos(100+20)/2cos(100-20)/2 =2cos(60)cos(40) =2*1/2*cos(40) =cos(40)cos20+cos100+cos140=cos40 + cos140 =2cos(40+140)/2cos(40-140)/2 =2cos90cos50 =2*0*cos50 =0



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