S10=a1(1-q^10)/(1-q)S5==a1(1-q^5)/(1-q)所以(1-q^10)/(1-q^5)=1/21+q^5=1/2q^5=-1/2所以S15/S5=(1-q^5)/(1-q^5)=1+q^5+q^10=1-1/2+1/4=3/4
