是sinx +cosx≤ m 恒成立吧
∵sinx +cosx=√2sin(x+π/4)
∵sinx +cosx≤ m
即√2sin(x+π/4)≤ m
即（√2sin(x+π/4)）max≤ m
∴√2≤ m
∴m≥√2