y=2sin3x
sin3x=y/2
3x=arcsin(y/2)
x=arcsin(y/2) /3
当x=－π/6时 y=2sin(－3π/6)=-2sinπ/2=-2
当x=π/6时 y=2sin(π3/6)=2sinπ/2=2
∴反函数是
y=arcsin(x/2) /3 x∈[-2,2]