There is a line $A B=8 \mathrm{~cm}$; at the midpoint $C$ of this line, we erect a perpendicular, the length of which is $C D=2.2 \mathrm{~cm}$; what is the circumference of the circle that can be drawn through points $A, B$ and $D$?
If the radius of the circle is $r$, then $r^{2}=\overline{A C}^{2}+(r-C D)^{2}$, from which

$$
r=\frac{\overline{A C}^{2}+\overline{C D}^{2}}{2 C D}=\frac{16+4.84}{4.4}=4.736 \mathrm{~cm}
$$

and thus $K=29.759 \mathrm{~cm}$.

(Júlia Simonyi, Eger.)

Number of solutions: 24.