f(x)是以4为周期的周期函数
x属于(-2,1],f(x)= 2^(x-2) + 1/5
x属于（-1,0）,f(x)=2^x+1/5
x=0,f(x) = 0
x属于（0,1）,f(x)=-2^(-x)-1/5
x属于[1,2),f(x)=-2^(x-2) - 1/5
x=2,f(x) = 0
4< log2 20 < 5
f( log2 20 )
= f(log2 20 -4)
= -2^[(log20-4-2)]-1/5
= -20/64-1/5
= -41/80