π |
3 |
π |
3 |
2π |
3 |
再由 cos2A+cos2C=
1+cos2A |
2 |
1+cos2C |
2 |
1 |
2 |
1 |
2 |
4π |
3 |
=1+
1 |
2 |
1 |
2 |
4π |
3 |
4π |
3 |
=1+
1 |
4 |
| ||
4 |
1 |
2 |
π |
3 |
再由A<
π |
3 |
π |
3 |
π |
3 |
1 |
2 |
π |
3 |
π |
3 |
故 1+
1 |
2 |
π |
6 |
故选D.
1 |
2 |
3 |
2 |
π |
3 |
π |
3 |
2π |
3 |
1+cos2A |
2 |
1+cos2C |
2 |
1 |
2 |
1 |
2 |
4π |
3 |
1 |
2 |
1 |
2 |
4π |
3 |
4π |
3 |
1 |
4 |
| ||
4 |
1 |
2 |
π |
3 |
π |
3 |
π |
3 |
π |
3 |
1 |
2 |
π |
3 |
π |
3 |
1 |
2 |
π |
6 |