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# Equation of a Circle

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## Mr Mattock

on 24 September 2016

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#### Transcript of Equation of a Circle

Equation of a Circle
Starter
Work out how far each of these points is from the origin (0, 0).

(a) (0, 5)

(b) (-5, 0)

(c) (3, 4)

(d) ( 5 , 2 5 )
Examples
Describe the graph with equation
x
+
y
= 7

Find the equation of a circle centred on the origin that passes through the point (2, 6).
The equation of a circle
The equation of a circle centred on the origin is given by

x
+
y
=
r

where
r
is the radius of the circle.
L.O. - Understand the format for the equation of a circle, and work with circle equations.
Starter
Work out how far each of these points is from the origin (0, 0).

(a) (0, 5)

(b) (-5, 0)

(c) (3, 4)

(d) ( 5 , 2 5 )

All of them are 5 units away from the origin.
2
2
2
2
2
Examples
Describe the graph with equation
x
+
y
= 7

Circle, centre (0, 0) with radius 7.

Find the equation of a circle centred on the origin that passes through the point (2, 6).

2 + 6 = 40 so equation is
x
+
y
= 40
2
2
2
2
2
2
Activity
Page 373 of Higher tier GCSE book
Plenary
What do you notice?

Does it always happen like this?
Plenary
What do you notice?
It is r
Does it always happen like this?
Yes.
(10, 0)
2
Activity