cotb=根号5.sina/sinb=sin(a+b)=-3/5,求sin2b
问题描述：
cotb=根号5.sina/sinb=sin(a+b)=-3/5,求sin2b
答
cotb=√5 tanb=√5/5
sinb/cosb=√5/5
sinb=√5/5cosb
解得到cosb=√30/6,sinb=√6/6
sin2b=2sinbcosb=√5/3