Problems from Area and perimeter of a circumference

Development:

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

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

Solution:

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

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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

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

Solution:

$$P=62,83$$ cm

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