f'(x)=ln2*2^(3x-x^2).
f'(x)是个复合函数.
所以只需求g(x)=3x-x^2的单调区间了.
g(x)在（负无穷,3/2）上单增,在(3/2,正无穷)上单减,根据同增异减原理,那么
f(x)在（负无穷,3/2）上单增,在(3/2,正无穷)上单减.