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a1=4,q=穿讲通准1/2

问题补充说明:求具体步骤

a1=4,q=穿讲通准1/2

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