设t=3x-3π/4,
则f(x)=2sin(3x-3π/4)=g(t)=sin(t)
则t的定义域就是[-π/2,-π/2].
g(t)=sin(t)在[-π/2,-π/2]上是单调增,
t=3x-3π/4关于x也是单调增,
所以f(x)单调增.