k=-1,cosx=-2不符合要求
k≠0
(k+1)sin^2x-4cosx+3k-5=0
(k+1)cos^2x-4cosx+4-4k=0
[(k+1)cosx-2(1-k)](cosx-2)=0
cosx-2≠0,
(k+1)cosx-2(1-k)=0
cosx=2(1-k)/(k+1),-1