令t=(1/2)^x,t>0.
f(t)=-t^2+4t+5=-(t-2)^2+9,则f(t)在(0,2)上单调递增,在（2,正无穷）上单调递减.
又因t(x)在R上随x单调递减,则f(x)在(-1,正无穷）上单调递减,在（负无穷,-1）上单调递增.
f(t)=-(t-2)^2+9(t>0),则f(t)