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# Circle equations of centre (0,0). Examples include draw the graph of x^2 + y^2 = 9.

The equation of a circle that has its centre as the origin (0,0), has the equation:

**x² + y² = r²**

(r stands for

the radius of the circle)

Drawing this graph is actually quite simple to do:

1) First, square root the number on the right hand side of the equation as this will give you the radius of the circle.

2) Next, open your compass to this radius you have just worked out and draw your circle from the centre of the coordinate grid.

**Example 1**

Draw the graph of x² + y² = 9

Since the equation starts off with x² + y² you know that its a circle with centre (0,0). Now, follow the two steps above:

1) First, square root the number on the right hand side of the equation as this will give you the radius of the circle.

√9 = 3.

So the circle has a radius of 3.

2) Open your compass to 3 units and draw your circle (shown above).

**Example 2**

Draw the graph of x² + y² = 9²

Since the equation starts off with x² + y² you know that its a circle with centre (0,0). Now, follow the two steps above:

1) First, square root the number on the right hand side of the equation as this will give you the radius of the circle.

√(9²) = √81 = 9.

So the circle has a radius of 9.

2) Open your compass to 9 units and draw your circle (shown below)

For some more maths help, take a look at these other hubs:

Hot maths topics for the 2011 GCSE maths foundation calculator paper.

2011 GCSE Maths Foundation Exam. 5 top topics to revise to help you get a grade C (non-calc paper)

How to complete the square on a quadratic equation (completing the square).

Circle equations. Using the centre and radius to write down the equation of a circle.