Area and perimeter of a circumference
If we say that the length of the element $r$ is $31$ cm, what is the area that encloses the circumference?
Let $r=31$ cm. We have, according to the formula $A= \pi \cdot r^2$
$$A=\pi \cdot 31^2=3019,07 \ \mbox{cm}^2$$
$$A=3019,07 \ \mbox{cm}^2$$
If we say that the length of the element $D$ is $20$ cm, what is the perimeter of the circle?
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
$$P=2\cdot \pi \cdot r=2 \cdot \pi \cdot 10 = 62,83 \ \mbox{cm} $$
$P=62,83$ cm