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