sin78°+sin18°+cos138°=sin(60°+18°)+sin18°+cos(60°+78°)=sin60°cos18°+cos60°sin18°+sin18°+cos60°cos78°-sin60°sin78°=[(√3)/2]cos18°+(1/2)sin18°+sin18°+(1/2)cos78°-[(√3)/2]sin78°=[...