I2.1 The volume of a sphere with radius $r$ is $36 \pi$, find $r$.
I2.2 If $r^{x}+r^{1-x}=4$ and $x>0$, find $x$.
$\begin{array}{l}\frac{4 \pi}{3} r^{3}=36 \pi \\ r=3 \\ 3^{x}+\frac{3}{3^{x}}=4 \quad \text { It is straight forward by guessing } x=1 \text { ) } \\ \left(3^{x}\right)^{2}-4 \cdot 3^{x}+3=0 \\ \left(3^{x}-1\right)\left(3^{x}-3\right)=0 \\ 3^{x}=1 \text { or } 3 \\ x=0 \text { (rejected, as } x>0) \text { or } 1\end{array}$