设公本比为q,
a3-a1=a1q^2-a1=15
a1(q^2-1)=15 (1).
a2-a1=(a1(q-1)=5.
a1(q-1)=5 (2).
(1)/(2),得:q+1=3.
q=2,a1=5.
S5=a1(q^5-1)/(q-1).
=5*(2^5-1)/(2-1).
=5*(32-1).
=5*31.
∴S5=155.(1)/(2),得:q+1=3.？？？？？？？？？？a1(q^2-1)=15.(1)a1(q-1)=5 (2).(1)/(2):[a1(q^2-1)]/[a1(q-1)=15/5.a1(q+1)(q-1)/a1(q-1)=3.左边分子分母约去a1(q-1)后,得:q+1=3.q=3-1=2.