| sin80°−sin20° |
| cos80°−cos20° |
=
| sin(60°+20°)−sin20° |
| cos(60°+20°)−cos20° |
=
| sin60°cos20°+cos60°sin20°−sin20° |
| cos60°cos20°−sin60°sin20°−cos20° |
=
| ||||||
|
=
| ||||||
−
|
=
| sin120°cos20°+cos120°sin20° |
| cos120°cos20°−sin120°sin20° |
=
| sin140° |
| cos140° |
=tan140°．
故答案为140°．
| sin80°−sin20° |
| cos80°−cos20° |
| sin(60°+20°)−sin20° |
| cos(60°+20°)−cos20° |
| sin60°cos20°+cos60°sin20°−sin20° |
| cos60°cos20°−sin60°sin20°−cos20° |
| ||||||
|
| ||||||
−
|
| sin120°cos20°+cos120°sin20° |
| cos120°cos20°−sin120°sin20° |
| sin140° |
| cos140° |