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Direct common tangents
Important facts
Transverse Common tangents
What do we understand by co-axial circles and limiting points?
When do two circles intersect orthogonally?
Related Resources
There can be four common tangents to two circles. The tangents can be either direct or transverse.
When the two circles neither intersect nor touch each other, there are four common tangents.
But when they intersect there are two common tangents, both of them being direct.
(i) The direct common tangents to two circles meet on the line of centres and divide it externally in the ratio of the radii. (ii) The transverse common tangents also meet on the line of centres and divide it internally in the ratio of the radii.
• When one circle lies completely inside the other without touching, there is no common tangent.
• When two circles touch each other internally 1 common tangent can be drawn to the circles.
• When two circles intersect in two real and distinct points, 2 common tangents can be drawn to the circles.
• When two circles touch each other externally, 3 common tangents can be drawn to the circles.
• When two circle neither touch nor intersect and one lies outside the other, then 4 common tangents can be drawn.
All this infromation along with the related conditions can be summarized as follows:
Number of Tangents Condition
4 common tangents r_{1} + r_{2} < c_{1}c_{2}
3 common tangents r_{1} + r_{2} = c_{1}c_{2}
2 common tangents |r_{1} - r_{2}| < c_{1}c_{2 }< r_{1} + r_{2}
1 common tangents |r_{1} - r_{2}| = c_{1}c_{2 }
no common tangents c_{1}c_{2 }< |r_{1} - r_{2}|
The lines represented in red in the figure given below are external common tangents and those represnted in blue are internal common tangents.
P is the point of intersection of two direct common tangents to the circles with centres C_{1} and C_{2} and radii r_{1}, r_{2} respectively. C_{1}A_{1}, C_{2}A_{2} are perpendiculars from C_{1} and C_{2} to one of the tangents (figure given below) ∴ ΔPC_{1}A_{1} and ΔPC_{2}A_{2} are similar
(C_{1}P)/(C_{2}P) = (C_{1} A_{1})/(C_{2} A_{2} ) = r_{1}/r_{2} i.e. P is a point dividing C_{1}C_{2} externally in the ratio r_{1} : r_{2}. For finding direct common tangents of two circles, find the point P dividing the line joining the centre externally in the ratio of the radii. Equation of direct common tangents is SS_{1} = T_{2} where S is the equation of one circle.
P is the point of intersection of two transverse tangents to two non-intersecting circles with centres C_{1} and C_{2} and radii r_{1}, and r_{2} respectively. Then P lies on the line joining the centres. C_{1}A_{1} and C_{2}A_{2} are perpendiculars from C_{1} and C_{2} to one of these tangents. (Figure given below)
Since triangles C_{1}A_{1}P and C_{2}A_{2}P are similar. So (C_{1} P)/(C_{2} P)=(C_{1} A_{1})/(C_{2} A_{2} )=r_{1}/r_{2} i.e. P divides the line joining C_{1} and C_{2} internally in the ratio r_{1 }: r_{2} Equation of transverse Common tangents is SS_{1} = T_{2} where S is the equation of one of the circle.
Distance between the centres of two circles should be greater than sum of the radii of the circles to keep a possibility of transverse common tangent which is clear from the figure given above.
Note:
Transverse tangents or Direct common tangents always meet on the line joining centres of the two circles.
Can we find the equation of the line joining the centres of two circles?
If S_{1} = 0 and S_{2} = 0 are two given circles, then S_{1} – S_{2} = 0 gives the equation of a line related to both of them. This line can give various results in varying conditions of relative positioning of two circles. (i) Radical Axis of the two circles: Radical axis of the two circles is defined as the line from each point of which, tangents of equal length are drawn to the two circles. (see figure given below).
In case of three circles radical point can be defined as the point where radical axis of three circles taken two at a time intersect. It is the point from which tangent of equal length can be drawn to all the three circles.
If S ≡ x^{2} + y^{2} + 2gx + 2fy + c S’ ≡ x^{2} + y^{2} + 2g’x + 2fy’ + c’ Then equation of radical axis of two circles S = 0 and S’ = 0 is given by S = S’ i.e. x^{2} + y^{2} + 2gx + 2fy + c = x^{2} + y^{2} + 2g’x + 2f’y + c’ 2(g – g’)x + 2(f – f’)y + (c – c’) = 0 (ii) If the circles touch each other then the above equation gives the common tangent at the point where the circles touch each other. It is also the Radical axis of the two circles.
(iii) If the two circles intersect then above equation gives the equation of the common chord. Here also the common chord is also the Radical axis of the two circles.
Two circles with centres C_{1}(x_{1}, y_{1}) and C_{2}(x_{2}, y_{2}) and radii r_{1}, r_{2} respectively, touch each other.
(i) Internally: If |C_{1} C_{2}| = |r_{2} – r_{1}| and the point of contact is ((r_{1} x_{2 }- r_{2} x_{1})/(r_{1}+r_{2} ) , (r_{1}y_{2 }- r_{2}y_{1})/(r_{1}+ r_{2})). (ii) Externally: If |C_{1} C_{2}| = |r_{2} – r_{1}| and the point of contact is ((r_{1}x_{2 }+ r_{2} x_{1})/(r_{1}+ r_{2}) , (r_{1} y_{2}+r_{2} y_{1})/(r_{1}+r_{2})).
A system of circles every pair of which has the same radical axis is called a coaxial system.
The centres of circles of a coaxial system, which are of zero radiuses, are called the limiting points o the coaxial system. Let the equation of a system of coaxial circles be x^{2} + y^{2} + 2gx + c = 0 Where g is a parameter and c is a constant. It’s radius √(g^{2}-c)and centre is (–g, 0) If g^{2} – c = 0 or g = +√c, then radius become zero and for these two values of g we have two circles of zero radius whose centres are (± √c, 0).
These circles of zero radius are just points and according to definition given above are the limiting points of the co-axial system.
If the system of circles are non intersecting then c is positive and these limiting points are both real.
If c = 0, points of intersection are coincident to (0, 0) i.e.circle touch each other at (0, 0). Limiting points coincide at (0, 0).
The angle of intersection between two curves intersecting at a point is the angle between their tangents drawn at that point. The curves are said to be intersecting orthogonally, if the angle between their tangents are common point is a right angle.
Consider two circles
S_{1} ≡ x^{2} + y^{2} + 2g_{1}x + 2f_{1}y + C_{1} = 0
S_{2} ≡ x^{2} + y^{2} + 2g_{2}x + 2f_{2}y + C_{2} = 0
They intersect at point P such that tangent PT_{1} and PT_{2} are at right angle (see figure right side) Since radius of a circle is perpendicular to the tangent. So C_{1}P and C_{2}P are also perpendicular. ⇒ (C_{1}C_{2})^{2} = (C_{1}P)^{2} + (C_{2}P)^{2} ⇒ (g_{1} – g_{2})^{2} + (f_{1} – f_{2})^{2} = r_{1}^{2} + r_{2}^{2} g_{1}^{2} + g_{2}^{2} – 2g_{1}g_{2} + f_{1}^{2} + f_{2}^{2} – 2f_{1}f_{2 }= g_{1}^{2} + f_{1}^{2} – C_{1} + g_{2}^{2} + f_{2}^{2} – c^{2} ⇒ 2g_{1}g_{2} + 2f_{1}f_{2} = C_{1} + C_{2} which is the required condition for the orthogonal intersection of two circles.
At what angle do the circles shown in figure intersect?
From the triangle C_{1}C_{2}P it is clear that angle θ can be written as: cos θ = (r_{1}^{2}+r_{2}^{2}-d^{2})/(2r_{1} r_{2} ) Hence, this is the required angle.
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