若tan a= - 1/3,求(1)COS(派/2+a) (2)3/(sina乘COSA)

tana=sina/cosa=-1/3cosa=-3sina(cosa)^2+(sina)^2=110(sina)^2=1sina=根号10/10或-根号10/10cos(Pai/2+a)=-sina=-根号10/10或根号10/10(2)3/(sinacosa)=3/(sina*(-3sina))=-1/(sina)^2=-10