Question 551080
<pre>
Substitute the value of the radius for r in

x² + y² = r²

For instance, to get the answer to 
</pre>
find the equation of a circle centered at the origin and having radius 5
<pre>
just substitute 5 for r and get

x² + y² = 5²

and then do one more step, change 5² to 25

x² + y² = 25

That's the equation.

That's all there is to it!

Edwin</pre>