椭圆y^2/12+x^2/8=1 焦点坐标A（0,-2）,C（0,2）,
B在椭圆上
BA/sinC=BC/sinA=AC/sinB=t
BA=t*sinC BC=t*sinA AC=t*sinB
（sinA+sinC）/sinB
=(BC+BA)/AC
=2a/2c
=a/c
=2√3/2
=√3