对于无穷等比数列a,aq,aq^2,...,其和S=a/（1-q）①奇数项和比偶数项多27,即(a+aq^2+aq^4+...)-(aq+aq^3+...)=(a-aq)+(aq^2-aq^3)+...=a(1-q)+a(1-q)q^2+...它是以a(1-q)为首项,q^2为公比的无穷等比数列,所以S1=a(1-q...