Circles could touch internally,
Externally,
Or not at all
Example
Do the circles with equations
and
touch ?
The first circle, C1, has centre A(-3 , 2)
and radius
The second circle, C2,has centre B(6, -1)
and radius
Using the distance formula,
Since AB = r1 +r2, the circles touch externally.
Example
Do the circles with equations
and
touch ?
The first circle, C1, has centre A(4, 2)
and radius r1 = 3
The second circle, C2,has centre B(5, 2)
and radius r2 = 2
Using the distance formula,
Since AB = r1 - r2, the circles touch internally.
Example
In the diagram below, the point C(-1,4) is the point
of contact of the two circles.
Given that the radius of the larger circle is twice
the size of the radius of the smaller circle, find
the equation of the small circle.
Since C is point of contact for both circles,
distance AC must be the radius of the large circle
and CB the radius of the small circle.
Using the distance formula,
Given that AC =2CB
The centre of the smaller circle can be found by
finding the distances along and up for the larger
circle, halving these distances (since half the radius)
and applying to point C.
This is called the stepping out method.
From the distance formula for AC, (above)
the step out is 4 along and 4 up.
The next step out must be 2 along and 2 up
from point C.
Which gives the point B(1,6)
The equation of the small circle is
Alternatively,using the section formula
gives
The equation of the small circle is