求函数y=cosx-3/cos+3的值域

由题知,
y*(cosx+3)=cosx-3
则：cosx=-3(y+1)/(y-1)
由cosx的取值范围知：-1≤cosx≤1
所以-1≤-3(y+1)/(y-1) ≤1
由-3(y+1)/(y-1)≥-1解得y≥-2
由-3(y+1)/(y-1)≤1解得y≤-1/2
则其值域为：-2≤y≤-1/2