(2sin78°-sin18°)/(2cos78°+根号3sin18°)
=(2sin（60°+18度）-sin18°)/[2cos(60°+18°)+根号3sin18°]
=（2sin60°cos18°+2cos60°sin18°-sin18°）/(2cos60°cos18°-2sin60°sin18°+根号3sin18°)
=（根号3cos18°+sin18°-sin18°）/(cos18°-根号3sin18°+根号3sin18°)
=根号3cos18°/cos18°
=根号3