sina+cosa=根号3／3
(sina+cosa)^2=(根号3／3)^2,
(sina)^2+(cosa)^2+2sinacosa=1/3,
1+2sinacosa=1/3,
2sinacosa=-2/3,sinacosa=-1/3
tana+cota=sina/cosa+cosa/sina=[(sina)^2+(cosa)^2]/(sinacosa)
=1/(sinacosa)=1/(-1/3)=-3