问题补充说明:求具体步骤
a1q^2=1........石灯是哪程在...1
a1(1-q^3)/(1-q)=7...承汉亮双味械游优..........2
由2式360问答得
a1*(1-q)(1能推少刑亲城思害持+q+q^2)/(1-q)=7
a1*(1+q+q^2)=7
a1+a1q+a1q^2=7
a1+a1q+1=7
a1+a1q=6.............3
1式除以帮出练读3式
a1q^2/(a1+a1q)=1/6
q^2/(1+q)=1/6
6q^2=振1+q
6q^2-q-1=0
(2q-1)(3q+1)=0
2q-1=0, q=源状岁东民介1/2
3q+1=0, q=-1/3
当q=1/2时,a1=1/q^2=1/(1/4)=4
当q=-1/3时,a且1=1/q^2=1/(1/食社提二联适计阶9)=9