**Question 9**In a geometric progression, the first term is 18 and the common ratio is

*r*.

Given that the sum to infinity of this progression is 21.6, find the value of

*r*.

[ Remember the formula for sum to infinity ${S}_{\infty}=\frac{a}{1-r},\text{}\text{}\text{\hspace{0.17em}}\text{\hspace{0.17em}}-1<r<1$]

**Solution:**