I4.3 In Figure 1 , there are eight positive numbers in series. Starting from the $3^{\text {rd }}$ number, each number is the product of the previous two numbers. Given that the $5^{\text {th }}$ number is $\frac{1}{r}$ and the $8^{\text {th }}$ number is $\frac{1}{r^{4}}$. If the first number is $s$, find the value of $s$.
Let the $2^{\text {nd }}$ number be $a$, then other numbers are as follows:
$s$
$s^{2} a^{3}=\frac{1}{r}$ $\square$
$\square$ $s^{8} a^{13}=\frac{1}{r^{4}}$
$s^{8} a^{13}=\frac{1}{r^{4}}$
$s^{2} a^{3}=\frac{1}{r}$ $\qquad$
$s^{8} a^{13}=\frac{1}{r^{4}}$ $\qquad$
$(2) \div(1)^{4}: a=1$
Sub. $a=1$ into (1): $s^{2}=\frac{1}{r}=\frac{1}{2} ; s>0 \Rightarrow s=\frac{1}{\sqrt{2}}$