Shortcut Trick
When two circles touch each other externally, the angle subtended by the direct common tangent at the point of contact is always 90°.
∴ In ΔAPB, ∠APB = 90°.
Using the angle sum property of a triangle in ΔAPB:
∠ABP = 180° − (∠APB + ∠PAB)
∠ABP = 180° − (90° + 65°) = 25°
∴ The correct answer is 25°.
Alternate Method
Given:
Two circles touch each other externally at point P.
AB is a direct common tangent touching the circles at A and B.
∠PAB = 65°
Formula/Property Used:
- Tangents drawn from an external point to a circle are equal in length.
- Angles opposite to equal sides in a triangle are equal.
- The sum of angles in a triangle is 180°.
Calculations:
⇒ Let the common tangent at the point of contact P intersect the direct common tangent AB at point T.
⇒ For the left circle, TA and TP are tangents drawn from the external point T.
∴ TA = TP
⇒ In ΔTAP, since TA = TP, the angles opposite to these sides are equal:
⇒ ∠TPA = ∠TAP = 65°
⇒ Similarly, for the right circle, TB and TP are tangents drawn from the external point T.
∴ TB = TP
⇒ Since TA = TP and TB = TP, we have:
⇒ TA = TB = TP
⇒ In ΔAPB, the median PT is equal to half of the opposite side AB (since T is the midpoint of AB and TP = AB/2).
⇒ This property holds true only in a right-angled triangle where the hypotenuse is AB.
∴ ∠APB = 90°
⇒ Now, in ΔAPB, using the angle sum property:
⇒ ∠PAB + ∠ABP + ∠APB = 180°
⇒ 65° + ∠ABP + 90° = 180°
⇒ 155° + ∠ABP = 180°
⇒ ∠ABP = 180° − 155° = 25°
∴ The correct answer is 25°.
Additional Information
Angle Subtended by Direct Common Tangent
When two circles touch each other externally, the direct common tangent always subtends a right angle (90°) at the point of contact of the two circles.
Length of Direct Common Tangent (DCT)
For two circles of radii R and r with distance d between their centers, the length of the DCT is given by √(d2 − (R − r)2). When they touch externally, d = R + r, simplifying the formula to 2√(R × r).
Number of Common Tangents
When two circles touch externally, there are exactly 3 common tangents: 2 direct common tangents and 1 transverse common tangent.