[tex]\begin{gathered} T_{10\text{ }}=ar^{n-1} \\ T_{10\text{ }}=ar^{10-1}\text{= 63.41} \\ ar^9\text{= 63.41}-----------(1) \\ T_{21\text{ }}=ar^{20} \\ ar^{20}\text{= 618.16}-----------(2) \\ \frac{T_{21}}{T_{10}}\text{ = }\frac{618.16}{63.41} \\ \\ \frac{ar^{20}}{ar^9}\text{ = }\frac{618.16}{63.41} \\ r^{11}\text{ = 9.74} \\ \text{But from} \\ r^{11}\text{ = 9.74} \\ r^{9\text{ }}\times r^2\text{ = 9.74} \\ 63.41\text{ }\times r^2\text{ = 9.74} \\ r^2\text{ =}\frac{9.74}{63.41} \\ r^2\text{ = 0.1536} \\ r\text{ = }\sqrt[]{0.1536} \\ r\text{ = }0.391 \end{gathered}[/tex]
