Pole  $P$  koła o promieniu równym  $r$  jest równe:

$P=\pi r^2$  

$a)$  

Wiemy, że  $P=9\pi {\text{cm}}^2$ , zatem

$9\pi {\text{cm}}^2=\pi r^2\mid :\pi$  

$9{\text{cm}}^2=r^2$  

$r=3\text{cm}$  

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$b)$  

Wiemy, że  $P=\frac{4}{25}\pi {\text{cm}}^2$ , zatem

$\frac{4}{25}\pi {\text{cm}}^2=\pi r^2\mid :\pi$  

$\frac{4}{25}{\text{cm}}^2=r^2$  

$r=\frac{2}{5}\text{cm}$  

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$c)$  

Wiemy, że  $P=17{\text{cm}}^2$ , zatem

$17{\text{cm}}^2=\pi r^2\mid :\pi$  

$\frac{17}{\pi }{\text{cm}}^2=r^2$  

$r=\sqrt{\frac{17}{\pi }}\text{cm}$