x∈[0,π/3],-3x+π/4∈[-3π/4,π/4],
当x=,π/4时,-3x+π/4=-π/2,f（x）取最小值-2
当x=0时,-3x+π/4=π/4,f（x）取最大值√2
f（x）在定义域内是连续的
f（x）=2sin（-3x+π/4）的值域是[-2,√2]