4cos50°-tan40°=_．
问题描述：
4cos50°-tan40°=______．
答
4cos50°-tan40°=4sin40°-tan40°
=
4sin40°cos40°−sin40° cos40°
=
2sin80°−sin(30°+10°) cos40°
=
2cos10°−
cos10°−1 2
sin10°
3
2 cos40°
=
cos10°−3 2
sin10°
3
2 cos40°
=
3
cos(30°+10°) cos40°
=
．
3
故答案为：
．
3