一个等比数列的第二项和第三项分别为 $10$ 和 $8$。

求这个数列的无穷和。

Answer all questions. You should use a calculator where appropriate.

You must show all necessary working clearly; no marks will be given for unsupported answers from a calculator.

Give non-exact numerical answers correct to 3 significant figures, or 1 decimal place for angles in degrees, unless a different level of accuracy is specified in the question.

请回答所有题目。适当时应使用计算器。必须清楚展示所有必要的计算过程；仅给出计算器答案而没有过程，不予评分。除非题目另有精度要求，非精确数值答案保留 3 位有效数字，以度为单位的角度保留 1 位小数。

用第三项除以第二项求公比，再倒推首项。检查公比的绝对值小于 $1$。

The common ratio is $8/10=4/5$ and the first term is $10/(4/5)=25/2$. Since $|4/5|<1$, the sum to infinity is