sin40°乘于（tan10°－根号3）=sin40°*(sin10°/cos10°-sin60°/cos60°)=sin40°[(sin10°cos60°-cos10°sin60°)/cos10°cos60°]=sin40°*(-sin50°)/(1/2*cos10°)=-2sin40°sin50°/cos10°=-2sin40°cos40...