Problems from Area and perimeter of a circumference

Development:

Let $r=31$ cm. We have, according to the formula $A=\pi \cdot r^2$

$$A=\pi \cdot {31}^2=3019,07{\text{cm}}^2$$

Solution:

$A=3019,07{\text{cm}}^2$

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Development:

In this case we do not know the value of $r$, but we know the value of $D$. We also know that $D=2\cdot r$, so the radius is half the diameter. Therefore we have $r=10$ cm.

Using the following formula $P=2\cdot \pi \cdot r$ and knowing the radius is $10$ cm we have

$$P=2\cdot \pi \cdot r=2\cdot \pi \cdot 10=62,83\text{cm}$$

Solution:

$P=62,83$ cm

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