y=-sin²θ-2msinθ-2m-1
y=-sin²θ-2msinθ+m²-m²-2m-1
=-（sin²θ+2msinθ+m²)+m²-2m+1-2
=-(sinθ+m)²+(m-1)²-2
∵-(sinθ+m)²≤0
∴(m-1)²-2＜0
∴(1-√2)＜m＜(1+√2)