q^5 = (a6+a7+a8+a9+a10) / (a1+a2+a3+a4+a5) = - 3/27 求出q -27 = a1+a2+a3+a4+a5= a1(q^5-1)/(q-1) 求出a1 Sn = a1(q^n -1)/(q-1) 求出Sn
