用辅助角公式：
2√2sinx +cosx=3sin(x+θ)=(6m-9)/(4-m)
sin(x+θ)=(2m-3)/(4-m)
因 |sin(x+θ)|≤1
故 |2m-3|/|4-m|≤1
解得：-1≤m≤7/3