Common Tangents


 
 
Concept Explanation
 

Common Tangents

Common Tangents: If a line is tangent to two circles in the same plane then the line is called the common tangent. If the two circles are on the same side of the tangent then it is called a direct tangent otherwise if the two circles are on the other side of tangent it is called an indirect tangent,

The number of common tangents for concentric circles is zero.

The number of common tangents for two circles touching internally is one.

The number of common tangents for two intersecting circles is  two.

The number of common tangents for two circles touching externally is three out of which two are direct tangent and the third is an indirect tangent.

The number of common tangents for two non intersecting and not touching circles is four. out of which two are direct and two are inditrect.

Illustration: Two circles touch externally at P and a common tangent touches them at A and B. Prove that

(i) The common tangent at P bisects AB                                            (ii) AB subtends a right angle at P

Solution : Let the common tangent at P touches the common tangent AB at T

        AT = PT         (1)   [ Length of tangent from point T are equal]

        BT= PT          (2)   [ Length of tangent from point T are equal]

From (1) and (2) We get     AT = BT

Hence T bisects AB

In large Delta APT        large angle TAP = angle TPA                 (3)   [ angle opposite equal sides are equal]

In large Delta BPT        large angle TBP = angle TPB               (4)    [ angle opposite equal sides are equal]

  adding (3) and (4)

large angle TAP+ angle TBP = angle TPA+angle TPB=angle APB        (5)

  In large Delta APB

large angle TAP+ angle TBP +angle APB = 180^0         [ Angle sum property]

large angle APB +angle APB = 180^0                         [Using Eq (5) ]

large Rightarrow ;;;2angle APB = 180^0                   large Rightarrow ;;;angle APB = 90^0

Hence AB subtends angle of  right angle at P

 
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