原式=2tan70°[cos10°*1/2+（√3/2）sin10°]-2cos40°
=2tan70°(cos10°sin30°+cos30°sin10°)-2cos40°
=2tan70°sin40°-2cos40°
=2cot20°sin40°-2cos40°
=2(cos20°/sin20°)2sin20°cos20°-2cos40°
=4(cos20°)^2-2[2(cos20°)^2-1]
=4(cos20°)^2-4(cos20°)^2+2
=2.