设t=-x^2+3x-1/4,
t=-x^2+3x-1/4=-(x-3/2)^2+2
因为-(x-3/2)^2≤0,所以-(x-3/2)^2+2≤2.
y=(2/3)^t是减函数,
所以(2/3)^t≥(2/3)^2=4/9,
函数值域是[4/9,+∞）.