∫x/2^xdx = ∫x2^(-x)dx =-(1/ln2) ∫xd2^(-x)=-(1/ln2) x2^(-x) + (1/ln2) ∫2^-xdx= -ln2x2^(-x) -1/(ln2)^2 2^(-x) |0, 正无穷大=1/(ln2)^2既然这个积分收敛，那么原来级数也收敛
