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Two circles C1 C2 externally touching and have radii a, b. AB is direct common tangent. A lies on C1 and B lies on C2 . Find the radius of circle C which touches C1 and C2 externally and also line AB
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amitnrw: 1/√r = 1/√a + 1/√b

Answers

Answered by amitnrw
11

Given : Two circles C1 C2 externally touching and have radii a, b. AB is direct common tangent. A lies on C1 and B lies on C2 .  circle C touches C1 and C2 externally and also line AB

To find :  radius of circle C

Solution:

Refer the attached picture :

CX ║ AB

=> BX =  a

& C₂X  = b - a

C₁X = AB

C₁C₂  = a + b

(C₁C₂)² = C₁X² + C₂X²

(a + b)² = AB² + (b - a)²

=> a² + b² + 2ab = AB² + b² + a² - 2ab

=> AB² = 4ab

=> AB = 2√ab

Similarly

if we take Circle C radius as  R

Then Taking C₁ & C Circles

AP = 2√aR

Taking C₂ & C Circles

BP = 2√bR

AB = AP + BP

=> 2√ab  = 2√aR  + 2√bR

Dividing by 2√abR

=> 1/√R  = 1/√b  + 1/ √a

=>  1/√R  = (√a + √b)/√ab

=> √R = √ab /  (√a + √b)

=> R  =  ab/ (√a + √b)²

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