cosa=-√2/3,a属于(pai/2,pai),
所以sina=√7/3
2/sin2a-cosa/sina
=1/(sinacosa)-cosa/sina
=(1-cosacosa)/(sinacosa)
=7/9/(-√14/9)
=-√14/2
cosa=-√2/3,a属于(pai/2,pai),
所以sina=√7/3
2/sin2a-cosa/sina
=1/(sinacosa)-cosa/sina
=(1-cosacosa)/(sinacosa)
=7/9/(-√14/9)
=-√14/2