What is the area of a circle whose equation is (x-3)² + (y + 1) ² =81?
can someone explain this to me? im not really looking for solution i want to understand that to do some exercises on my own, me with that

Okay, first, given the equation, we need to find out what the radius of the circle is. Let us state the general equation of a circle: 



Where is the centre of the circle. In this case, we don't need to know the centre. Just the radius. 

Let us start by converting the equation into standard for, which I typed above. Divide both sides by 81. 



Great! We now know the radius of the circle. It is  because it is the bottom fraction. Now we know that the radius is 9. 

So now lets input this into the area of circle formula: 

 

Now we insert our radius. 

 

 

You can convert that into a decimal if you wish. 

Hope this helped!

~Cam943, Moderator