an = a1q^(n-1)
case 1:if |q|>1
lim(n->无穷)an 不存在
case2:if q=1
an =a1
lim(n->无穷)an =1
case3:if q=-1
lim(n->无穷)an 不存在
case 1:if |q|无穷)an=a1