cos10°

tan20°

+

3

sin10°•tan70°-2cos40°
=

cos10°

tan20°

+

3 sin10°

tan20°

-2cos40°
=

2sin(10°+30°)

tan20°

-2cos40°
=

2sin40°

tan20°

-2cos40°
=4cos²20°-2cos40°
=4cos²20°-4cos²20°+2
=2．