Enter one known value of a circle and calculate the area, circumference, radius or diameter. You will get solution with stepbystep explanation.

To calculate missing value in circle, based on one known value, you need to remember just three formulas.
F1:  $$ A = r^2 \cdot \pi $$ 
A = area C = circumference r = radius d = diameter 
F2:  $$ C = 2 \cdot r \cdot \pi $$  
F3:  $$ d = 2 \cdot r $$ 
The diagram at the right shows when to use each of these formulas.
Find circumference of a circle when the area is 20.
To solve this problem we first need to find radius $r$, by using formula $ \color{blue}{A = r^2 \pi} $ (see the diagram).
$$ \begin{aligned} & \color{blue}{A = r^2 \pi} \\ & 20 = r^2 \pi \\ & r^2 = \frac{20}{\pi} \\ & r^2 = \frac{20}{3.14} \\ & r^2 = 6.37 \\ & r = \sqrt{6.37} \\ & r = 2.52 \end{aligned} $$Now we will find circumference using the formula $\color{blue}{C = 2r\pi} $.
$$ \begin{aligned} & \color{blue}{C = 2 r \pi} \\ & C = 2 \cdot 2.52 \cdot 3.14 \\ & C = 15.82 \end{aligned} $$Please tell me how can I make this better.
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