(2014?北京)如图,在△ABC中,∠B=π3,AB=8,点D在边BC上,且CD=2,cos∠ADC=17.(1)求sin∠BAD;(
(2014?北京)如图,在△ABC中,∠B=π3,AB=8,点D在边BC上,且CD=2,cos∠ADC=17.(1)求sin∠BAD;(2)求BD,AC的长....
(2014?北京)如图,在△ABC中,∠B=π3,AB=8,点D在边BC上,且CD=2,cos∠ADC=17.(1)求sin∠BAD;(2)求BD,AC的长.
展开
4个回答
展开全部
(1)在△ABC中,∵cos∠ADC=
,
∴sin∠ADC=
=
=
=
,
则sin∠BAD=sin(∠ADC-∠B)=sin∠ADC?cosB-cos∠ADC?sinB=
×
-
×
=
1 |
7 |
∴sin∠ADC=
1?cos2∠DC |
1?(
|
|
4
| ||
7 |
则sin∠BAD=sin(∠ADC-∠B)=sin∠ADC?cosB-cos∠ADC?sinB=
4
| ||
7 |
1 |
2 |
1 |
7 |
| ||
2 |
3
|