sin(a+nπ)+sin(a+nπ)/sin(a+nπ)cos(a-nπ)(n∈z)
当n=2k ,k∈z时
原式=sina+sin(a)/sin(a)cos(a)=sina+1/cos(a)
当n=2k+1 ,k∈z时
原式=-sina-1/cos(a)