幂级数变成x/4+1/2(x/4)+1/3(x/4)+…+1/n(x/4)+…，设S(x)=z+(1/2)x+(1/3)x+…+(1/n)x+…，逐项求导，S′(x)=1+x+x+…+x+…=1/1-x(-1<x<1)，s(x)==-ln(1-x)，所求幂级数的和函数为-1n(1-x/4)=-ln(4-x)+ln4。