When $\frac{\sin 990}{\cos 540}+\frac{\sin 780}{\cos 750}+\frac{\sin 390}{\cos 780}=k(\tan 405-\tan 360)$ is defined, then $k=$

sin990=sin(11.90+0)=-cos0=-1

cos540=cos(6.90+0)=-cos0=-1

sin780=sin(8.90+60)=sin60

cos750=cos(8.90+30)=cos30=sin60

sin390=sin(360+30)=sin30

cos780=cos(8.90+60)=cos60=sin30

tan405=tan(4.90+45)=tan45

tan360=tan0