Question

sin 17°. Cos 73° - cos 13° cos 21 - Six 4. Sin 86°​

Answer 1

cos17° cos73 - sin13° cos21 - cos4° cos86° ​=0

cos17° cos73=1/2(cos(17+73)+cos(17-73))=1/2(cos(90)+cos(-56))=1/2cos56=1/2cos(90-34)=1/2sin34

sin13° cos21=1/2(sin(13+21)+sin(13-21))=1/2(sin34-sin8);

cos4° cos86° ​=1/2(cos(4+86)+cos(4-86))=1/2(cos90+cos82)=1/2cos(90-8)=1/2sin8

____________

1/2sin34-1/2(sin34-sin8)-1/2sin8=0