sin78°-sin42°=sin(60°+18°)-sin(60°-18°)= 2cos60°sin18°=sin18°sin66°-sin6°=sin(60°+6°)-sin6°=sin60°cos6°+cos60°sin6°-sin6°=sin(60°-6°)=sin54°∴sin78°-sin66°-sin42°+sin6°=sin78...