| 2sin20°cos20°cos40°cos80° |
| 2sin20° |
| 2sin40°cos40°cos80° |
| 22sin20° |
| 2sin80°cos80° |
| 23sin20° |
∴cos20°•cos40°•cos80°=
| 1 |
| 8 |
| sin160° |
| sin20° |
| 1 |
| 8 |
| sin(180°−20°) |
| sin20° |
| 1 |
| 8 |
又∵cos60°=
| 1 |
| 2 |
∴cos20°•cos40°•cos60°•cos80°=
| 1 |
| 2 |
| 1 |
| 8 |
| 1 |
| 16 |
故选C
| 1 |
| 4 |
| 1 |
| 8 |
| 1 |
| 16 |
| 1 |
| 32 |
| 2sin20°cos20°cos40°cos80° |
| 2sin20° |
| 2sin40°cos40°cos80° |
| 22sin20° |
| 2sin80°cos80° |
| 23sin20° |
| 1 |
| 8 |
| sin160° |
| sin20° |
| 1 |
| 8 |
| sin(180°−20°) |
| sin20° |
| 1 |
| 8 |
| 1 |
| 2 |
| 1 |
| 2 |
| 1 |
| 8 |
| 1 |
| 16 |