3^x在定义域上的范围是[1/3,9]，则值域为[-8,2/3]
y=f(x)=(1/4)^x-(1/2)^x+1(x∈[-3,2])t=(1/2)^x>0y=(t-1/2)^2+3/4t=1/2,x=1f(-3)=64-8+1=57f(1)=3/4f(2)=1/16-1/4+1=13/163/4<=y<=57
