Question

已知0<a<π/2,π/2<B<π,且tana/2=1/2,sin(a B)=5/13，求sinB的值

Answer 1

tana=2(tana/2)/[1-(tana/2)^2]
=2*1/2/[1-1/4]
=1/(3/4)
=4/3

sina=4/5,cosa=3/5

sin(a+b)
=sinacosb+cosasinb
=4/5cosb+3/5sinb
=4/5cosb+3/5√(1-cos^2 b)
5/13=4/5cosb+3/5√(1-cos^2 b)
5/13-4/5cosb=3/5√(1-cos^2 b)
25/169+16/25cos^2b-8/13cosb=9/25(1-cos^2 b)
25/169+cos^2b-8/13cosb-9/25=0
cos^2b-8/13cosb-896/4225=0
cos^2b-8/13cosb+16/169-16/169-896/4225=0
(cosb-4/13)^2-1296/4225=0
(cosb-4/13-36/65)(cosb-4/13+36/65)=0
(cosb-20/65-36/65)(cosb-20/65+36/65)=0
(cosb-56/65)(cosb+16/65)=0
cosb=56/65或cosb=-16/65
因为90°<b<180°
所以cosb=-16/65
sinb=63/65