因为sina+3cosa=0
所以sina=-3cosa
代入sin^2a+cos^2a=1得cos^2a=1/10
(sina+3cosa)^2=0
sin^2a+6sinacosa+9cos^2a=0
8cos^2a+3sin2a+1=0
则sin2a=-3/5
π+2kπ<2a<2π+2kπ
π/2+ο<a<π+kπ(k∈Z)
所以a在第二,第四象限
因为sina+3cosa=0
所以sina=-3cosa
代入sin^2a+cos^2a=1得cos^2a=1/10
(sina+3cosa)^2=0
sin^2a+6sinacosa+9cos^2a=0
8cos^2a+3sin2a+1=0
则sin2a=-3/5
π+2kπ<2a<2π+2kπ
π/2+ο<a<π+kπ(k∈Z)
所以a在第二,第四象限