sin²13°+cos17°-sin13°cos17°=(1-cos26°)/2+(1+cos26°)/2-sin13°cos17°=1+(cos26°-cos34°)/2-sin13°cos17° =1-sin[(26°+34°)/2]sin[(26°-34°)/2]-[sin(13°+17°)+sin(13°-17°)]/2 （积化和...