cosa=-1/5,且5/2π<a<3π,那么sina/2=

问题描述:

cosa=-1/5,且5/2π<a<3π,那么sina/2=

cosa=1-2(sina/2)^2=-1/5,
(sina/2)^2=3/5,
5/4π<a/2<3π/2,
a/2在第三象限,sina/2所以sina/2=(√15)/5.